Natural Fancy Colors (7.37)
Natural fancy colors can be extremely valuable, even in the very common yellow hue. A vivid fancy yellow in a very large size could easily be worth more than the house you live in. At the other extreme, light fancy yellows, are very moderate in value because of their abundance. A high saturation level is very important to overall value because of its rarity and general appeal. A fancy vivid yellow is a very valuable color, but add a touch of brown and its value drops to a fraction.
GIA considers D-Z as the colorless through light tones on their diamond color grading scale which would include faint, very light and light where as anything past Z will enter the fancy color scale starting with Fancy Light. A GIA Colored Diamond Grading Report is applicable for K-Z colors excluding yellow and brown and any color from Fancy Light through Fancy Deep.
The GIA Colored Diamond Grading Scale:
Faint
Very Light
Light
Fancy Light
Fancy
Fancy Intense
Fancy Vivid
Fancy Dark
Fancy Deep
The Fancy Intense and Fancy Vivid will generally demand higher prices the the Fancy Dark and Fancy Deep. The most valuable colored diamonds will fall into the "rare colors" category discussed in subsection 7.28.
The combination of the following will generally increase value:
Rare Color
Larger Carat Weight
Higher Clarity
Excellent Cut Specs
Purity of Hue (lack of modifying colors)
Higher Saturation
The least valuable colors will be light gray, light yellow and light brown. Add a modifying color and this will generally lower value such as Fancy Intense Brownish-Yellow versus an Intense Yellow.
The above images are courtesy of: https://fancycolordiamond.net/