A white square on a black background will look larger than a black square on a white background. However, a medium gray square will look the same size on either a white background or a black background.

A white square on a black background results in a black afterimage, while a black square on a white background results in a black afterimage. However, a gray square on a gray background results in no afterimage.

If you put a gray square on a larger background of pure color, the gray will take on the semblance of the background color's complementary color. On yellow, it will look gray-violet; on orange, bluish; on red, greenish.

The human eye seeks out balance, which explains the above. Medium gray creates an equilibrium which the eye does not seek to correct.

Gray can be mixed from black and white. It can also be mixed from two complementary colors and white. Also, you can get gray from mixing several colors, provided they contain the three primary colors -- red, yellow and blue -- in the right proportions.

Two or more colors can be considered harmonious if their mixture results in a neutral gray. Thus, any color combinations which to not result in gray are considered expressive or discordant.

Gray can also be spelled grey.