How is diamond color evaluated ? White diamonds are compared to the set of master stones with your pre-graded. The best way to compare the diamonds while they are upside down up against a white background such as  plain white piece of paper is best to use Northlight daylight or a  fluorescent desk lamp .

Basically what I’m getting at is important to have a standardized viewing environment because different light sources and different backgrounds can affect how the diamond looks well is being evaluated .

Some prestigious diamond rating laboratories use 2+ independent diamond evaluations an average out the color grade.

Opinions may vary from between some diamond graders.   Usually they’re all within a certain range .