How does diamond colour grading work?

Diamond colour grading measures how colourless a white lab-grown diamond is, or the hue, saturation, and intensity of a fancy-coloured stone. For white diamonds, IGI uses an alphabetical scale running from D (completely colourless) to Z (a visible yellow or brown tint). The D-to-Z scale applies only to white diamonds; fancy-coloured stones are assessed separately by identifying the dominant hue first, then grading its intensity across categories such as Faint, Intense, or Vivid.