A diamond’s color grade actually refers to the absence of color. In other words, diamonds that are white, containing little or no color, receive higher quality grades than those with noticeable color when grading white diamonds.
A chemically pure and structurally perfect diamond has no hue, like a drop of pure water, and consequently, a higher value. A D-to-Z diamond color-grading system measures the degree of colorlessness by comparing a stone under controlled lighting and precise viewing conditions to master stones stones of established color value.
Read about diamond clarity