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first off, I’d really appreciate any help. I have googled relentlessly to try to understand this, but I’m too stupid to fathom it. Please attempt to explain this to me in plain English if you can.

I have recently learned about how subpixels work in an RGB LCD display, and that there are 0-255 numbers that represent each color.

Now, as far as I understood in the past, different wavelength photons (like red and green making light appear yellow for example) could strike your eye cones and change the perceived color. But from what I understand, the 0-255 number for RGB represents the voltage of the subpixels and thus allows more or less light to pass through the twisted nematic crystal. So ultimately, it’s the amplitude (number of photons) that changes with red, blue, and green colors. This would mean that combining different amplitudes of a given wavelength for red, green, and blue could change the color you perceive…but how is that any different than brightness? Brightness increases the amplitude and therefore the amount of photons striking your eyes.

If I use a testing website and set the value of red to 200 for example(0,0 for green and blue), the color is darker. Not just less bright, but the red is actually darker. So that means it must be a different wavelength, right? But this doesn’t make sense since the subpixels can only adjust how much light makes it through the crystals.

Also, if I set the values to 255, 255, 255, it’s white, even at low screen brightness on my iPhone. But if I set the settings to 200, 200, 200, and turn the brightness to max on my iPhone, the color is gray, even though increasing brightness should make up for the subpixels letting less light in. But at 255, even at low brightness, the color stays white. So that must mean that amplitude (in terms of photons) isn’t the primary factor in colors right?

Sorry if this post is unclear. Any help with helping me to understand would be amazing! If you need more clarification, please let me know. I love to learn and this physics question is driving me nuts!

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  • $\begingroup$ What's the difference between brown and orange? The Technology Connections YouTube channel has a number of good videos on rgb color, and this one points out a problem in your methodology. I mean, brown and orange are clearly different, right? youtube.com/watch?v=wh4aWZRtTwU $\endgroup$
    – Sean E. Lake
    Commented Sep 8, 2022 at 7:04
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    $\begingroup$ This seems to be to do with the biology of colour perception, and the quirks of screen programming. You might try taking a look at our Biology, Medical Sciences and of course Psychology & Neuroscience stacks. Since you seem interested/knowledgeable about computer screens, I've found this post you might be interested in. $\endgroup$
    – Jiminy Cricket.
    Commented Sep 8, 2022 at 7:05
  • $\begingroup$ Thank you guys. $\endgroup$
    – theguineapigking
    Commented Sep 8, 2022 at 7:45
  • $\begingroup$ Here's a good starting point, en.wikipedia.org/wiki/Opponent_process if you want to learn more about how human color perception is supposed to work. $\endgroup$
    – Solomon Slow
    Commented Sep 8, 2022 at 12:58

2 Answers 2

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If I use a testing website and set the value of red to 200 for example(0,0 for green and blue), the color is darker.

(I assume you mean darker than 255,0,0.)

Not just less bright, but the red is actually darker.

I assume you mean by this that you perceive it as a different hue.

That's a matter of color psychology, not physics. On a normal display, 200,0,0 and 255,0,0 will produce the same spectrum of light; 200,0,0 just produces less of it per unit time. The light hits cones in your eye and causes them to fire in certain patterns. 200,0,0 makes them fire less rapidly than 255,0,0. Your brain tries to make sense of the world from that sensory data. It is "within its rights" to convert the lower cone firing rate into a different perceived hue. There's nothing in the laws of physics that requires any particular pattern of cone firing rates to result in the subjective perception of any particular color.

But if I set the settings to 200, 200, 200, and turn the brightness to max on my iPhone, the color is gray, even though increasing brightness should make up for the subpixels letting less light in. But at 255, even at low brightness, the color stays white.

This is psychology again. Your brain has a flexible idea of how bright "white" is, which is why a full moon can look white even though it's 400,000 times dimmer than the sun. You are somehow inferring the screen brightness from some other clue, such as the brightness of the status icons, or light leaking around the edge of the screen, or just your memory of having set it. If all of those clues were taken away, you wouldn't be able to tell the dimmed white from the gray.

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  • $\begingroup$ Thank you for your reply. So, I’ve learned in the past that mixing different wavelengths(hues) is what allows you to see different colored lights(like yellow). Are you saying that amplitude(number of photons at a given wavelength) can do that too, at said ratio between the RGB spectrum? Like more red photons than green will change the perceived color, compared to equal amounts of red and green? Thanks again for your help. It’s been confusing because I learned how this all works from bad sources. $\endgroup$
    – theguineapigking
    Commented Sep 8, 2022 at 7:44
  • $\begingroup$ @theguineapigking Changing the ratio of red and green will usually change the perceived hue. Multiplying the R, G, and B channels by the same factor usually changes the brightness and not the hue, but it can change the hue (someone in a comment mentioned the example of brown being dark orange). The point I wanted to make is that the determination of the hue happens in the incredibly complex neural circuitry of the brain, and not in the much simpler world of light waves, so there are no simple physical laws governing hues. $\endgroup$
    – benrg
    Commented Sep 8, 2022 at 7:59
  • $\begingroup$ Thank you so much for the help. So both amplitude (number of photons), wavelength, and ratio of red to green to blue can all change the perceived hue? Like if you have green and red both at 255, and in one scenario the red is a higher wavelength, and in another, it’s a lower wavelength (with the green wavelength remaining constant), that will also affect perceived hue, correct? I just want to make sure I understand. Thanks again $\endgroup$
    – theguineapigking
    Commented Sep 8, 2022 at 10:25
  • $\begingroup$ @theguineapigking Amplitude and R/G/B ratio yes. Changing wavelength... depends on what you mean. The red emitted by an RGB monitor must stimulate the cones in a certain way, dictated by an RGB standard, which for most monitors is sRGB. Shifting the wavelength will throw the monitor out of spec, so it's no longer sRGB. If that doesn't bother you, then yes, it's another way to change what you see. $\endgroup$
    – benrg
    Commented Sep 8, 2022 at 17:51
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The luminance that sensed by your eye is the brightness Y = 0.2126 R + 0.7152 G + 0.0722 B. It is actually the yellow color.

Similarly the white balance (the proportion of R,G,B ) for a peak white color is the same as that of the 50% grey color but the luminance is just 50% of the peak value, so you see grey rather than the white.

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