The Simultaneous Contrast Effect
The two center squares have the same color. Change the colors behind them to see their apparent contrast.
This Demonstration was inspired by the work of Josef Albers.
Based on: A. Buhler-Allen, "The Math of Art: Exploring Connections between Math and Color Theory," in Proceedings of the Bridges London Conference, (R. Sarhangi and J. Sharp, eds.), Hertfordshire, UK: Tarquin Publications, 2006 p. 517.
Here is an excerpt from Buhler-Allen's paper:
In the mid 1800s a chemist with a specialty in dyes, Michael Eugene Chevreul, introduced his book, The Principles of Harmony and Contrast of Colors . Chevreul noticed that a color's character changes; the same color can look lighter or darker depending on its juxtaposition to other colors. It is said that his "Law of Simultaneous Contrast" was conceived in the tapestry (Gobelin) factory where he noticed that the strength of the black depended on what colors were next to it.  M.E. Chevreul, The Principles of Harmony & Contrast of Colors, Faber Birren, first printed 1839