(noun) - Tone is a quality of color. It has to do with whether or not a color is warm or cold, bright or dull, light or dim and pure or "otherwise" (not wishing to to slap the perceived moral judgment of impure on a hapless color). You've most likely heard the phrase "Tone it down". In art, this means to make a color, or an overall color scheme, less vibrant. Conversely, "toning it up" can cause colors to pop out of a piece, sometimes to a rather startling extent.