How do experts assess a diamond’s color? Although many diamonds appear colorless, GIA gemologists can detect even the slightest presence of yellow or other hues. GIA established the color grading standard, called the GIA Color Scale, to indicate the absence or presence of color. The scale begins with the letter D, representing colorlessness, and continues with the increasing presence of color to the letter Z. Diamond graders use color-comparison diamonds, called masterstones, to determine a diam