Visual Analytics - An Interaction of Sight and Thought - JMP

Context affects what we see. This image illustrates the surprising effect that a simple
change in the lightness of the background alone has on our perception of color. The
large rectangle displays a simple color gradient of a gray-scale from fully light to fully
dark. The small rectangle is the same exact color everywhere it appears, but it doesn’t
look that way because our brains perceive visual differences rather than absolute
values, in this case between the color of the small rectangle and the color that
immediately surrounds it.
Among other things, understanding this should tell us that using a color gradient as the
background of a graph should be avoided.