The color grade given to a diamond is measured by the lack of color. Yellow and brown tones are a result of impurities in the nitrogen of the diamond.
In the laboratories, the color grade is determined by viewing the diamonds from the side, and comparing to a master set of diamonds graded from D-Z. D-F represents the “colorless” range, and G-J the “near colorless.” These are the most common color grades for the larger center diamonds purchased for engagement rings.
Each letter grade represents a “range” within that grade. Therefore, the same color grade in different diamonds may look like they have a different color. The less color a diamond has, the more rare and expensive it is.