One key role of the sensory nervous system is that of data reduction so the brain is not overloaded with too much data.  One way this happens is that differences are highlighted and "sameness" is neglected when the retina signals the brain (the eyes discard about 99% of the information falling on the retina before transmitting what's left to the brain).  The important stuff are contrast and color differences, and motion.  The filtering out of sameness results, for example, in the false-color after-images you see after staring intently at something for awhile. 

This is a general strategy in the nervous system.  There's a well known analog to it in how we sense temperature.  Put one hand in hot water and the other in cold.  Then put both together in room temperature water, and each hand will inform your brain that that one temperature feels very different.