Gradient Bar Illusion
The presence of the gradient background makes it seem as though the rectangle is also a gradient, but in actuality it is one color.
 OpenProcessing. "Optical Illusion #2 (Gradient Illusion)." (Feb 8, 2016) smtp.openprocessing.com/sketch/168577.
"Gradient Bar Illusion"
Wolfram Demonstrations Project
Published: February 15 2016