Diamond color actually means the lack of color. The highest color grade is D, representing colorless, and continues down the alphabet, with increasing presence of color.
Typical range for Color is between D and H.
Diamonds found in most jewelry run from colorless to near-colorless, with slight hints of yellow or brown. Most people find it very difficult (if not impossible) to tell the difference between one color grade to another. The one difference people can see is in the price, which can be significant. GIA’s color-grading scale for diamonds is the industry standard. The scale begins with the letter D, representing colorless, and continues with increasing presence of color to the letter Z, or light yellow or brown. The less color a diamond has, the higher its color grade. Each letter grade has a clearly defined range of color appearance. Diamonds are color-graded by comparing them to stones of known color under controlled lighting and precise viewing conditions.
The reason why the GIA color grading starts with the letter 'D' because the creators of the GIA Color Scale wanted to start fresh, without any association with earlier systems, they chose to start with the letter D—a letter grade normally not associated with top quality.