This was a discussion I had with a couple of my friends in in school. The question is this -
If I see a color different from you, would we ever know the difference?
If that one sentence is not clear enough, let me go with an example. If what looks red to you looked green to me, and green looked something else, would we, or can we, ever come to know of this difference? And if we would not, or can not, are we sure such a thing is not actually happening? Are we sure we all see the same red?
The question came from, and the entire discussion started from Vivek. But the actual cause of trouble was something else. It was this day when he has (he claimed) a dream where he saw a new different color. And, though incredible, he said he could still picture it. But there was no way of telling me what it looked like. How do you define a new color?
Technically speaking, color is the impression produced in our eyes when the light of certain wavelength(s) falls on it. An object appears the color of light it emits or reflects. And the effect produced by a certain type (type - wavelength and other properties) of light rays always (probably, by and large) produces the same effect. So the first time in our life we perceive the wavelengh of 490-560nm, and wish to describe this sensation, someone tells us its the color ‘green’!
What actually happened in the above process is, we, as a human civilization, have mapped light wavelenghts to conventions, not perception. An object that emits rays in ~530nm, is ‘green’, because it looks similar to another previosly seen (and identified) object. Not because it produces a certain perception in the eyes.
Now if I were to have the kind of sensation in my eyes, while watching a, say green leaf, that happens to you when you see red, I would be actually seeing the color differently from you. But the first time I saw a green leaf, I would have to be made to call it (intialized), as described above, green only. And probably, I would never come to know of this.
Another friend came up with a counter arguement. The difference could be established, he said, based on the perception of brightness or deepness, which is how we describe our perception. We do feel sometimes that blue is soothing and yellow is sharp. This response of uneasyness on certain colors would differ if I were to see colors different from you. That arguement looked partially valid. We do have ways to describe color perception qualitatively, though it does not depend only on wavelength. There are several other factors (luminosity etc) that make yellow sharp and blue soothing. And variations in those other factors can actually make sharp colors look soothing and vice-versa. We do have easier shades of yellow. Moreover, what if the difference in color perception remain in the same range of physiological response of the colors? That is, what if I see one soothing color (say deep blue) as another (green)?
I had another arguement. Any such variation in perceptions, I said, should happen across the entire range of the color spectrum, and within the visible spectrum. This means that in my eyes, every color should be substituted with another color, if such a difference is there. If I see greens as red, I should see reds as something else. If this was not so, I would not be able to distinguish objects of different colors. That is a well known diability called color blindness. So far I was right. Then I made an incorrect assumption, that any such perception variation should occur linearly across the color spectrum.
The color spectrum is an orders visible colors of light based on wavelenghts. It is Red, Orange, Yellow, Green, Blue, Voilet. So say for example, if Red were to appear Yellow to me, and Yellow as Blue, then by this order, what would blue and voilet appear? They should become invisible, as per me_in_those_days. That was a mistake. Color perception is a complex electrostatic + biological phenomonon, and cannot be assumed to follow this logic.
The discussion ended at that point, but I am still partially convinced that it is quite possible different people see the same colors differently. But I never could see a new color in dreams. I’ll wait :)