What term describes the lightness or darkness of a color?

"Value" is the term that describes the lightness or darkness of a color. The value of a color helps to define a form and to create the illusion of depth in an image.

In subtractive color, such as an artist using pastels or paints, adding black or white changes the value of a color while reducing the color's saturation. An artist uses shading to reduce a color's value and highlights to increase a color's value, generating spatial illusions on an otherwise 2-dimensional surface. Through skillful use of value manipulation, an artist is capable of adding a sense of mass, volume, light and distance to the figures within the image.