A truly colorless diamond is extremely rare. Most diamonds possess varying degrees of yellow or brown and small, subtle differences in color can make a substantial difference in value. Although increasing shades of yellow can reduce the value of a diamond this does not necessarily reduce its beauty. If a diamond is well cut, the diamond's refraction and dispersion often will disguise certain degrees of coloration.
Unless a diamond is a fancy color (or any color other than colorless to light yellow or brown), The color grading system for diamonds uses the letters of the alphabet from D through Z, with being the most colorless and therefore the rarest and most valuable, and having the least color within the normal range, and being the least valuable, all other factors being equal. A diamond color is determined by looking at it under controlled lighting and comparing them to the Gemological Institute of America color scale, which is based on a set of diamonds of known color. The diamond is viewed face down and reassessed face-up for the most accurate and consistent grade.
