How does the colored diamond grading system differ from the D-to-Z diamond grading system?

GIA’s D-to-Z color grading system is based on the relative absence of color. GIA’s Fancy colored diamond color grading is based on the presence of color. GIA describes color in terms of hue (the color), tone (relative lightness or darkness), and saturation (intensity). Hue (like pink) is modified by a “Fancy-grade” term (Fancy Light, Fancy, Fancy Intense, Fancy Deep, Fancy Vivid or Fancy Dark) which describes the effect of both tone and saturation.