Diamond color evaluation is based on the absence of color- the less color the higher their value. Diamond color is graded on a scale from D-Z and measures the degree of colorlessness. Many of these color distinctions are so subtle that they are invisible to the untrained eye. These distinctions, however, make a big difference in diamond quality and price. Generally, when comparing color between two diamonds, the diamonds need to be at least two color grades apart to even begin to see a difference.