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As a kid, I was taught that that blue and yellow make green, yellow and red make orange, and red and blue make purple - forming the subtractive color wheel. As an adolescent I was taught that blue and green make cyan, green and red make yellow, and red and blue make magenta - forming the additive color wheel. Somewhere in that time I was also taught that the colors of the rainbow are ROY G. BIV.

That last one is the only one that makes much sense to me in physics because perceivable light is not somehow a cycle, but rather a tiny segment of the EM spectrum.

Why do the colors at the top and bottom of the spectrum seem to be mixable? Is there anything in nature that indicates this should be the case? Is this just a psychological or physiological phenomenon?

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    $\begingroup$ possible duplicate of Why does light of high frequency appear violet? $\endgroup$
    – user4552
    Commented Aug 30, 2013 at 14:37
  • $\begingroup$ It is physiological, the receptors of light at the retina of the eye.en.wikipedia.org/wiki/Color . It so happens that the spectrum frequencies correspond to the mixed colors we see through our eyes, though the color we see may not be of a single spectrum frequency. $\endgroup$
    – anna v
    Commented Aug 30, 2013 at 14:38
  • $\begingroup$ @annav: Your comment explains how the trichromatic theory leads to a continuous space of colors, but it doesn't address the question, which is about why a linear segment of the EM spectrum seems to wrap around into a circle in color space. $\endgroup$
    – user4552
    Commented Aug 30, 2013 at 15:00
  • $\begingroup$ @BenCrowell I guess then I do not understand the question. JohnBerryman can you give an image/link of this color wheel? $\endgroup$
    – anna v
    Commented Aug 30, 2013 at 16:49
  • $\begingroup$ @annav Ben feels the core of the question is "why does violet look redish" rather than "why do yellow and blue make green". google.com/… $\endgroup$
    – kaine
    Commented Aug 30, 2013 at 17:08

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Light wavelengths are on a linear scale, but humans only measure "color" by the relative power in three regions of this linear spectrum. A whole gamut of relative weights of these three color components is possible. The possible colors (hue and saturation, normalizing out intensity) we can perceive can therefore be represented as a triangle in a plane, which is often simplified to a circle.

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  • $\begingroup$ This explanation doesn't work. The fact that a planar region like a triangle is topologically equivalent to a circle doesn't tell us anything about whether the boundary of the region in this particular example maps onto a linear segment of the EM spectrum. You can't explain this correctly without the Hering-Hurvich-Jameson opponent processing theory, as discussed in my answer to the question that this one duplicates. $\endgroup$
    – user4552
    Commented Aug 30, 2013 at 14:56
  • $\begingroup$ @Ben: Read what I said more carefully. By sampling the magnitude of a function in only three places, and the resulting 3 samples can each range from 0 to 1, you can't distinguish whether the function is linear or circularly repeating. You are making this way more complicated than necessary. No, you don't need a bunch of long-winded fancy explanations for this. $\endgroup$
    – Olin Lathrop
    Commented Aug 30, 2013 at 18:13
  • $\begingroup$ I understand your argument. I disagree with it. I added some material to my answer here to discuss this idea: physics.stackexchange.com/a/66845/4552 . See the paragraph beginning "It is trivially true that ..." $\endgroup$
    – user4552
    Commented Aug 30, 2013 at 18:45
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You see color based on which of your light receptors in your eye detect that particular wavelength of light and to what degree. This is largely based on the size of the receptors but also on a lot of factors I don't understand.

Say: Receptor A reacts most light with wavelength a. Receptor B reacts most light with wavelength b.

Did you say that out loud? good!

Now Receptor A will react just a little with b and the closer b is to a, the more A will react.

If we see wavelength a as yellow and b as blue, there is some wavelength c between a and b which will trigger A and B equally and we will arbitraily call that green. (Note each receptor does not quite react exactly like this but this is an overly simplified analogy.)

You can imagine now that if we trigger A and B with a and b in the exact proportions that A and B are triggered by c, we will see green. That is (by my understanding and I work with inorganic pigments for a living) my understanding why the color wheel works the way it does. As to why it is fashionable to shown blue next to orange, I have no idea.

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  • $\begingroup$ This is fine as an explanation of why the trichromatic theory can explain the continuous nature of color, but it doesn't address the OP's question, which is about why the physically linear EM spectrum wraps around to make a circle. $\endgroup$
    – user4552
    Commented Aug 30, 2013 at 14:58

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