The ideal diamond color should be like a drop of pure spring water. The absence of any yellow or brown tint of color will result in the most pleasing appearance.
The grading scale begins at "D" with a perfectly colorless diamond and proceeds through the alphabet as a tint of yellow or brown color becomes more noticeable. The grade of "D" is the most rare and, as a result, the most expensive. Gemological Services sells diamonds primarily in the D to J range.
D to F = Colorless (no tint of color visible)
G to J = Near colorless (minimal tint of color, difficult to see)
K to M = Light tint (light tint of color, easy to see)
N to Z = Noticeable tint (tint of color, very easy to see)
On rare occasions diamonds may naturally be found in a full spectrum of colors such as blue, red, and green. These fancy-colored diamonds can range from a subtle tint to an intense saturation of color. Fancy colored diamonds are extremely rare and extremely expensive.
Some diamonds naturally glow under ultraviolet light. This fluorescence can range from faint to very strong in appearance. Blue is the most common color of fluorescence, but it may also range from yellow to green in appearance. Although some consumers seek out a stone with strong blue fluorescence for its unique phenomenon, most diamond dealers consider fluorescence to have a negative impact on value. Blue fluorescence in a diamond will rarely affect its visual appearance, but yellow or green fluorescence most often will have a detrimental effect on the vibrancy of the diamond under normal lighting conditions.