Colors, wavelengths, brains

[amoken]

Why do red and blue mix to make purple? When you mix two materials (of the same type), one of which reflects red and one of which reflects yellow, the resulting material reflects orange, a middling wavelength. Why is it that when you mix red and blue materials, the resulting material reflects something we view as purple/violet/thereabouts, rather

Comments

[benfrantzdale]

Our brains only distinctly detect red, green, and blue (or bell curves centered on those wavelengths).

Red paint mixed with yellow paint doesn't make orange paint. It makes paint with a spectrum with a red peak and a yellow peak. Compare that with something truly orange and your eye will pick up some red and some yellow from both.

Since we only have three classes of receptors, purple is the only color we see that has more than one peek that we can detect, but if we had full spectral vision we would have as many colors as we have timbres for instruments.

[thegreatgonz]

Strictly speaking, it's our eyes, not our brains. Human eyes have 4 types of receptors- one for night vision, and one each for red, green, and blue daytime vision.

There are also many layers of processing between the eye and the bran, many of which are not at all understood, which convert the responses of the three receptors into a single percieved "color". Loosely speaking, our brain takes the weighted average of the three.

To the best of my knowledge, there is no perceptible difference between mixed red and yellow paint and something "truly orange"- our eyes and brains can't register that difference.

[iainuki]

Is this a question with an answer? It seems to me you're asking why we interpret certain signals from our visual receptors (triggering of receptors) as a particular color. That sounds like a hard philosophical question, relating physical events to qualia.

I need to think about this in additive color. Let me do the mappings:

red + blue -> magenta
green + red -> yellow
green + blue -> cyan

Additive color doesn't map onto the color order in the spectrum except for yellow.

For some reason, our brain interprets equal stimulation of red and blue cones as violet. I think it comes down to the problem I mentioned above.

[dragonmudd]

To complicate ianuki's answer further, I read about a study that showed that what colors you see is cultural, as well as physical. It basically had to do with if you had a name for the different color or not. For example, let's say I put up to pieces of fabric that were very close in color, like teal and a light green color of some sort. The study showed that people from some cultures would be able to pick out which one was light green and which one was teal very regularly, but people from a different culture were completely unable to tell them apart.

[amoken]

Actually, after that study came out there were like a billion incensed anthropologists and a famous counter-study by Berlin and Kay, among several others that cast serious doubt on that conclusion. It does not show that people can't tell them apart; it shows that people choose to categorize among the category names they've learned

[nuclear_eggset]

this sort of thing came up at my last eye appointment when the doc asked me to do a color accuity test and sort samples of color from one end of the spectrum to another. i did it in... oh, let's just say an inordinately short amount of time, and commenting on that, the optomotrist and I had an interesting discussion over how much of that is learned, not natural. i learned it because i paint and do other forms of visual art, and so quickly determining what colors are in a color (so to speak) becomes easy as you do it so often. the optomotrist noted that his experience with his patients demonstrated the same thing.