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Like if you put a red spotlight and a blue spotlight on the same area, the light ends up magenta. And if your light is purple and you have a yellow chair, the chair will appear black because the chair isn't reflecting any of those wavelengths of light.

Presumably, there are pigments that reflect wavelengths beyond the visible spectrum, if one could see them, would mixing a pigment that reflects the very shortest infra-red wave length and the very longest ulta-violet wavelength combined to create a secondary color that reflects both wavelengths, or would it turn muddy like mixing blue with orange?

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  • $\begingroup$ Related: physics.stackexchange.com/q/552840/123208 Also see en.wikipedia.org/wiki/UV_coloration_in_flowers which has numerous academic references. $\endgroup$
    – PM 2Ring
    Commented Jul 10 at 10:50
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    $\begingroup$ "Color theory" including topics like the color wheel and complementary colors is mostly nonsense. It is to our modern understanding of color and light as alchemy is to chemistry. Some of it is right, or right in limited contexts, but it's based on wrong understandings about how colors work, so it breaks down quickly under scrutiny. $\endgroup$
    – Vaelus
    Commented Jul 10 at 14:58
  • $\begingroup$ @Vaelus Color theory describes ratios of frequencies within the spectrum visible to us, and it's perfectly practical for that. $\endgroup$
    – Mentalist
    Commented Jul 12 at 10:17
  • $\begingroup$ Madeira Darling, your question reminds me of a similar question about how sound frequencies may relate to color. Here's an answer you might find interesting. $\endgroup$
    – Mentalist
    Commented Jul 12 at 10:18
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    $\begingroup$ Mixing "a red spotlight and a blue spotlight" onto "white" surface will turn out more white-grey than magenta. It depends on the exact "red" and "blue" involved. $\endgroup$
    – Pablo H
    Commented Jul 12 at 14:04

7 Answers 7

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"Color" is something that happens in your eyes and brain. We call one wavelength of light "red" and another "blue" because of the sensations that they cause in your brain. If you see both at the same time, you get the sensation of "purple." But, the purpleness of it is only in your brain. Your brain is where the "mixing" happens.* A spectrometer would simply tell you that it's seeing the two particular wavelengths at the same time.

"Invisible" wavelengths are so called because they produce no sensation at all in your eyes and brain. If two different invisible wavelengths reach your eyes, there can be no mixing to produce a visible color because the only color mixing that ever happens is in your brain, and the invisible wavelengths don't cause your eyes to send any signals to your brain that can be mixed.

* For one model of how your brain interprets the signals from your eyes as colors, see https://en.wikipedia.org/wiki/Opponent_process


FWIW: In astronomy, you often see "false color" images where different invisible wavelengths recorded by a camera are "translated" to visible colors that were arbitrarily chosen by a person.

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  • $\begingroup$ I think it may be worth mentioning here that some materials may absorb one wavelength of light but re-emit it at another wavelength. e.g. UV light is famous for interacting with many materials in this way. So light beyond the visible spectrum can be relevant to color theory after all (though admittedly not in the exact way posed in the question). $\endgroup$ Commented Jul 12 at 23:43
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The color receptors in your eyes are sensitive to a particular range of wavelengths. When you're seeing an object, this could generally be a complicated collection of different wavelengths. These are then detected by three kinds of cones (color receptors) and your brain then interprets this mess. The sensitivity of these cones look as follows:

enter image description here

What does this mean? If light with a wavelength of 600 nanometers hits your eye, your cones will detect a lot of red, some green and no blue. Your brain does some processing and then correctly interprets this as orange. The same holds true for every other wavelength in this range. This is quite impressive. Imagine someone hands you 3 numbers (how much light the red green and blue receptors detect) and you have to figure out what the original color is!

You can also trick your eyes. Instead of sending actual orange light, i.e. light with a wavelength of 600 nanometers, you can also send a combination of green and red light. If you pick the proportions right, you will see this as orange light as well. By picking a mix of the three basic colors of light, you can generate any color that your eyes can see.

What happens when you mix red and blue? When you mix red and green your brain interprets that as light having a wavelength that is between red and green: orange or yellow. Similarly, when you mix green and blue you get cyan. So, if you mix red and blue you should get green, right?? Well no, if you look at the left (short end) of the spectrum, you see there's a portion that's suddenly more sensitive to red. You're brain interprets this wavelength as purple/violet. If you mix blue and red, the brain thinks it is in the short part of the spectrum. If you add a lot of red, your brain will make up a color. It doesn't have to get the right wavelength, it just has to produce discernible colors. If you want to know how violet light looks like: I have a laser pointer of 405 nm and it looks like a grayish, mostly blue shade of purple.

So to answer your question: light on the short end of the spectrum looks purple and light on the long end of the spectrum looks red. If you mix the two, you get some shade of purple/magenta.


EDIT: So, the plot thickens. Thanks to the comments for pointing that out. The image I showed you is not the sensitivity of your actual light cones. That would look like this:

enter image description here

Based on Dicklyon's PNG version, itself based on data from Stockman, MacLeod & Johnson (1993) Journal of the Optical Society of America A, 10, 2491-2521d http://psy.ucsd.edu/~dmacleod/publications/61StockmanMacLeodJohnson1993.pdf

Here S, M and L stand for short, medium and long wavelengths. The plot I showed earlier represents the sensitivity in something that's called the XYZ color space. To get these values, a number of people were shown two colors: one monochromatic (single wavelength) color, and one mix of colors given by 3 predetermined colors. The participants were asked to match the monochromatic color by mixing those 3 colors. From a more reputable source I got the following image:

enter image description here

Blaszczak, Urszula & Zajac, Andrzej. (2016). SELECTED METROLOGY PROBLEMS IMPLIED BY THE APPLICATION OF LED TECHNOLOGY IN LIGHTING. Informatics, Control, Measurement in Economy and Environment Protection. 6. 6-11. 10.5604/20830157.1212257.

Note that the normalisations can vary a bit between sources.

More info on this observer can be found https://royalsocietypublishing.org/doi/epdf/10.1098/rsta.1932.0005 and https://en.wikipedia.org/wiki/CIE_1931_color_space#CIE_standard_observer.

So to summarize, when looking just at receptors, you get responses that are centered at a single wavelength (2nd image). When you include the processing that's done in your brain/optical nerves, you get a bump near blue (1st, 3rd image). This means that if you observe violet light, it elucidates the same response in your brain as if you see red and blue light together.

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    $\begingroup$ You didn't answer the actual question though, which is what happens when you look at infrared and ultraviolet at the same time. Has nothing to do with purple and red. $\endgroup$ Commented Jul 10 at 22:17
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    $\begingroup$ Also note: You said, "purple/magenta," but where is magenta on the spectrum? Where is pink? What colors do you mix to get brown? What about gray? There are more colors that we can perceive than just the colors of the rainbow?. $\endgroup$ Commented Jul 11 at 2:56
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    $\begingroup$ "You can also trick your eyes. Instead of sending actual orange light, i.e. light with a wavelength of 600 nanometers, you can also send a combination of green and red light." I just have to point out that this is exactly what you're doing with the image! :D $\endgroup$
    – JiK
    Commented Jul 11 at 9:59
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    $\begingroup$ You're good. Not many people know about brown. Most pinks, on the other hand, are pale versions of purpleish colors, not pale red. My point though is that your answer might encourage someone to think that "all the colors of the rainbow" is a picturesque way to say "all colors," When, in fact, the rainbow is only a small subset of all colors. $\endgroup$ Commented Jul 11 at 10:31
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    $\begingroup$ "This is quite impressive." Actually not much. From programming POV it is a simple associative table mapping. You got three numbers and you remember that you was told (in childhood) that this combination is called "purple". A lot of memory is required for that but brain has enough capacity for that. The best thing is that the calibration is done individually so every human calls this color "purple" even if they may see it in very different ways. $\endgroup$ Commented Jul 11 at 11:29
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The short answer is: invisible light has no color.

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    $\begingroup$ Your "answer" is a comment. $\endgroup$
    – Mentalist
    Commented Jul 12 at 8:59
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    $\begingroup$ @Mentalist No, it isn’t. It’s short. $\endgroup$
    – my2cts
    Commented Jul 12 at 11:18
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    $\begingroup$ The same in more words? Colour is the sensation mediated by signals produced by retinal cells. Therefore, if electromagnetic radiation produces/causes no response from retinal cells, then no colour sensation happens. $\endgroup$
    – Pablo H
    Commented Jul 12 at 14:10
  • $\begingroup$ *For humans. We can make educated guesses on how other animals with higher bandwidth vision might experience infrared and ultraviolet based on their reninal physiology and analogies to human vision, but it's just a good guess, and it's different from species to species. $\endgroup$
    – Vaelus
    Commented Jul 12 at 17:35
  • $\begingroup$ @Vaelus It’s not really bandwidth but rather how many different photoreceptor types an animal has. Some have many more than we do, but I doubt that they have color theory. ‘ The most complex color vision system in the animal kingdom has been found in stomatopods (such as the mantis shrimp) having between 12 and 16 spectral receptor types thought to work as multiple dichromatic units’ en.m.wikipedia.org/wiki/Color_vision $\endgroup$
    – my2cts
    Commented Jul 12 at 21:56
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Each of our three photoreceptor (cones) outputs a single "number" (spike rate) to the upstream neurons, and these neurons calculate color using different levels fo complexity, for instance, from a specific color related to the relative excitations of the different receptors, if the color is isolated, to less physically related colors like when we see color illusions, usually by combining different colors in a physical pattern.

I think the closest answer to your question is that if the pigments you use, however strange they might be, excite teh receptors in a way similar to a color often seen, then you will likely perceive it as that color. But if it excites the three receptor in unusual ways, you might experience different colors, or at least color experiences, at this point it is just semantics. One example is neon colors, that were uncommon when I was young, so they were a new experience when I saw them for the first time.

Also, don't forget that different cultures separate the names of colors in different ways than we do. Do they perceive the colors differentently? (for instance blue and green are the same color in some cultures, such as the Greek and the Japanese).

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(This is more of a comment.)

Maybe you could try asking women around you!

More seriously, most known land animals having four distinct photoreceptors are birds, but it is possible that some humans, mostly (if not exclusively) female, might have four different receptors, the fourth being in the red/green region shifted towards infrared (IR). That would result in tetrachromatic vision allowing a slightly wider range perception towards the IR end of the visible spectrum. It is unclear if this can actually result in a different perception, especially when you consider the influence of culture, language (while being careful with linguistic relativism), and how we actually communicate on those impressions, as @pato-galmarini mentioned.

You might want to take a look at impossible colors addressing a different aspect of your question, in the visible range. To go further, you can try asking on philosophy SE, because it really starts to be a philosophy of the mind problem when you dig further into color perception.

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    $\begingroup$ The ability to have 4 photoreceptors is a genetic anomaly. Interestingly it only happens with women because when men get the same anomaly, they're color-blind. The existence of such women was theorized a long time ago, but it took a very long time to find an example because they're so exceedingly rare. en.wikipedia.org/wiki/… $\endgroup$ Commented Jul 12 at 18:50
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This isn't an answer. It's an attempt to make your question make more sense and reveal some weird insights on the way.

We should have been able to recover this question by adding additional information but we will quickly find a problem. Nevertheless there is a philosophical recovery at the end.

Attempt To Recover:

Humans typically have 3 types of cone cells $L,M,S$ which are maximally sensitive to the wavelengths in the size $564\text{nm}, 534\text{nm}, 420 \text{nm}$ respectively which we identify with blue, green, red.

The cone cells really act upon intervals of waves (actually they have DISTRIBUTIONS of sensitivity but we won't get into that), for example both $564\text{nm}$ and $565\text{nm}$ will activate an $L$ cone in a human eye.

So the name of the game might be,

given a new type of a human with more than $3$ cones each carrying an interval of activating wave-lengths. And a mixture of wavelengths that activates some of those cones, WHAT SINGLE wave length best approximates that same mixture of wavelengths.

But we quickly run into an issue.

Cones don't describe Color; Our Brain Does:

Consider the famous "purple". It is a mixture of red and blue. These are the lowest and highest wavelengths. The pure wave form of purple is referred to as "violet" and it only really activates the "blue"-cone. So why on earth do we think that mixing a long wavelength form of light with a somewhat short wavelength form of light creates a super long wavelength form of light?

There's no good way to logic yourself out of this using just the cones and picking the right "mixing function".

We have to accept that for different weighted "sets" of cone activation there exists a corresponding color in our brain.

The artists' color theory of color mixing then is really a mapping of how cones map to colors in our brain. And in actuality "color theory" is a function of the organism being studied, in our case humans. It is not an abstract idea that exists by itself. You can't "derive" a color theory as such. And so it does NOT make sense to speak of an artistic color theory for wavelengths of light outside what humans can see. Although you can certainly try to map it out for OTHER animals

So that really kills this question to some degree. But it was a nice question and I did enjoy thinking about it :)

So how to really recover question:

Pick a (not necessarily terrestrial) animal that is capable of seeing electromagnetic radiation. For this animal there will exist an artistic "color theory". For humans we get the standard one. I imagine the color theory of a Mantis shrimp is pretty wild.

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Fun facts:

  • We don't have absolutely monochromatic light. Well, for the purposes of the color vision, lasers qualify.
  • We don't have absolutely sharp wavelength cut-offs in reflection or transmission spectra. What we usually have is a superposition of Gauss-like curves.
  • The above combined: human eyes don't have sharp red and violet cutoffs either. While more or less official visible spectrum is between 380nm and 780nm, one could see dim reddish flashes of a 1000nm infrared TV remote in a dark room. People with older artificial eye lens can see as far as 300nm (and probably beyond, if one dares to experiment)
  • In the general case, long tails of strong near-IR and near-UV absorbtion bands manifest as blue or yellow colors, respectively. This is why water is blue and this is why most flowers are yellow.
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