Any Color in a diamond that is not in the usual spectrum of Colorless to Light Yellow, as indicated by a grade of D to Z.
Fancy colors include almost the full range of the visual spectrum. Various factors create the variety of colors in nature. Natural colors (rare and expensive in finer qualities) do occur, but they are more commonly created or enhanced by various treatments in a laboratory, including Irradiation and HPHT. In nature, small amounts of defects or impurities (about one per million of lattice atoms) color a diamond blue (boron), yellow (nitrogen), brown (lattice defects), green (radiation exposure), purple, pink, orange or red.
Here are a few examples of colored diamonds, next to colorless diamonds for comparison value ::