One of the “4 C’s” of diamond grading, color is graded on a letter scale with D being a colorless diamond, and Z being one with pale yellow color. The letters are grouped together in broader classifications:
G-J: Near Colorless
N-R: Very Light
Darker than Z takes you into the fancy color diamond range, which includes yellow, pink, blue, green, brown and black. These have their own color designations and are not graded on the D-Z scale.