Diamond Color Chart

This chart shows you the slight difference in yellow hues between the different color grades. D-to-F are considered colorless. 

The diamond color evaluation of most gem-quality diamonds is based on the absence of color. A chemically pure and structurally perfect diamond has no hue, and consequently, a higher value. GIA's D-to-Z diamond color grading system measures the degree of colorlessness by comparing a diamond under controlled conditions to master-stones of established color value.

GIA's diamond D-to-Z color grading scale is the industry's most widely accepted grading system. The scale begins with the letter D, representing colorless, and continues, with increasing presence of color, to the letter Z.

Source of information: http://4cs.gia.edu/