Question: Is it true that different people see colour differently? Eg. Could your blue be the same as my pink? If so, why is this and is it the same as the logic behind the blue+black/white+gold dress thing?

Susan Cartwright answered on 19 Jun 2015:

There are at least two different questions here.

1. “Given a certain wavelength of light, could my perception of its colour be different from yours, e.g. could your experience of “blue” (short wavelength light) be equivalent to my experience of “red” (long wavelength light)?”
This is incredibly difficult to test. We would agree that light of this wavelength is the colour we have been brought up to call “blue”, even though our actual perceptions differ. These perceptions are what philosophers call “qualia” (and argue over constantly). From a physicist’s point of view, it’s an undecidable argument – I can’t think of any experimental way to find out. I’m leaving it to the philosophers!

2. “Do different people disagree on what colour something is?”
Yes, in several ways. You see colour because you have three different colour-sensitive receptors in your retinas: a long-wavelength (“red”) one, a medium-wavelength (“green”) one and a short-wavelength (“blue”) one. These are rather misleading names: the sensitivities of the “red” and “green” receptors are actually very similar (they evolved from an accidental duplication of the same gene: most mammals have only two types of receptor), but the slight difference is enough to allow us to distinguish red from green (and the combined response gives yellow).
2a. In some people, almost all men (about 10x as many men as women), one of the colour receptor genes is either missing or mutated, and consequently one of the colour receptors is either missing or not working properly. This is “colour blindness” (the reason it’s so much more common in men is that the “red” and “green” genes are on the X chromosome: a woman would need to have faulty genes on both X chromosomes to be affected, but a man only has one X chromosome). How badly the colour vision is affected depends on which receptor is damaged, and whether it’s completely broken or just not working properly. The most common type is a malfunctioning (but not missing) green receptor, causing problems in distinguishing red from yellow and green.
2b. Some women inherit a malfunctioning red or green receptor from one parent, and a normal one from the other. They then have four different receptors (e.g. red, abnormal green, normal green, blue). There is some evidence that these women can distinguish colours in the red-yellow-green part of the spectrum more finely than people with normal 3-receptor colour vision. There are also more subtle individual variations: I know that I define as “blue” colours that my mother sees as “green” (I admit they are a greenish blue, but they definitely aren’t green as far as I’m concerned!). This is probably slight variations in the peak sensitivity of your three receptors – remember I got half of mine from my dad, so they aren’t necessarily the same as my mother’s.
2c. Our brains process colour perception a lot – it’s not simply a case of comparing signals from the 3 different receptors. In particular, we are good at adjusting for different colours of light. If your ambient light is blue rather than white, white paper (say) will reflect blue light, and should appear blue. In fact, your brain does its best to compensate for this effect: colours look the same to you under old-fashioned tungsten-filament light-bulbs as they do in daylight, even though the tungsten-filament light is much yellower than daylight. (Cameras don’t do this, which is why the colours of images taken without flash under normal room lighting often look a bit “off”.) This is the source of the blue+black/white+gold thing: people who saw it as blue and black, or blue and olive green, were processing the image assuming that the ambient light was white, whereas people who saw it as white and gold were (unconsciously) assuming that the ambient light was blue and “correcting” for this to give white instead of blue. Lighting a yellow colour with blue light results in black, since yellow stimulates the red and green receptors but not blue, so these people “corrected” the greenish black into gold.