Simultaneous contrast is the optical alteration of a color by a surrounding color. Different color combinations (hue, saturation, brightness) can results in color alterations such as making a color appear brighter, darker, more saturated, less saturated, and even make a color appear to be a different color.

In the image below, the gray square in each quadrant is exactly the same. However, due to simultaneous contrast it appears higher brightness in the blue quadrant and lower brightness in the yellow quadrant.