Why does the same color look different when it is surrounded by a lighter or darker color?

Question

http://langabi.name/blog/wp-content/images/blog/optical%20illusion.jpg

i tested it in photoshop

Answer 1

Simple:
You look with your eyes...but you see with your brain. Your brain will always adapt things your eyes see to something it can understand.

Your brain knows that one is a shadow and so the 'real' color must be lighter. It will not allow you to see it is the same.

Answer 2

It is not just he light or dark color next to it that affects it but the compliment of the color will change it also.
What is interesting is that there are many people who cannot see the same colors as you do. Science shows that women see more shades of red than men, and artists can see about 20 more gradations of values than the average person. The computer can show more colors than the printer can print. Color masters are much like piano tuners. Artist know that if they paint a painting in colors that have a more grayed look, and then pop a color into the art like light turquoise blue and put orange next to it, you eye will go to that spot immediately.
You can paint an interior of a house all the same color but it can look three shades darker in one room than another depending if the light changes from window light to fluorescent or lamp light, or if the carpet is dark or light and you would swear it was not the same paint.

Answer 3

Color interactions are valid and proven phenomena. The best physical explanation I've come across attributes it to the balance of chemicals in the rods and cones of the retina such that the temporary depletion of a particular color's chemical receptor through usage affects nearby rods and cones as well. There is considerable anecdotal evidence to support this idea (stare at a one color for a minute or two then close your eyes... you will see an afterimage of the shape in the original color's compliment, i.e. red/green, blue/orange, yellow/violet, and dark/light) and there is considerable research into photo/chroma reactive chemicals in vision, but to the best of my knowledge the idea itself is still hypothetical because of experimental limitations.