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

GIA’s D-to-Z colour grading system is based on the relative absence of colour. GIA’s Fancy coloured diamond colour grading is based on the presence of colour. GIA describes colour in terms of hue (the colour), 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.