If I would show someone a yellow object and ask them, "is this object yellow?" That person would say "yes".
But I could never know if my perception of the color yellow is the same as that other person's.
Because he or she could actually be seeing, what I know to be the color green.
But then tells me that its the color yellow because that has been taught to him or her from young age.
So how can you test if people are really seeing the same color?
(originally posted this question on physics.stackexchange but was advised to try it here) http://physics.stackexchange.com/questions/48731/how-can-you-test-what-color-different-people-perceive