How many blue tiles are on the top surface of the cube on the left? And how may yellow tiles on the top of the right cube? Four and seven, respectively, right? The correct answer for each question is none--both sets of tiles are gray.
The image on the right appears to be bathed in short-wavelength blue light, so our vision compensates by perceiving gray, a neutral shade, as longer-wavelength yellow. The opposite holds true on the left.
The fact that we can perceive colors consistently at all is astonishing, considering that our brains must take into account not only the color reflected from the object but also the color of the illumination and the context. Each parameter influences the frequency of the light as it reaches the eye, yet we still see blue as blue whether we're in brilliant sunshine, under fluorescent light, or looking through water. We are capable of distinguishing roughly 1 million colors.