"The less color, the higher the grade. Even the slightest hint can make a dramatic difference in value." (GIA)
Diamonds are graded upside down and with a "master set of stones."
FANCY COLORED DIAMONDS
"The grading of colored diamonds is conducted by a team of highly specialized GIA gemologists who examine the diamonds utilizing comparison masters. GIA’s nine-tiered rating system for color ranges from Faint to Fancy Vivid. Among the most famous colored diamonds are the Hope, a 45.52-carat blue diamond at the Smithsonian Institution – graded as Fancy Deep gray blue on the GIA scale – and the 41-carat Dresden Green Diamond, both fabled in their cultural history.
Naturally occurring colored diamonds, such as the Hope and Dresden, are very rare and therefore command top prices; diamonds that have been color-altered by laboratory treatment are less rare, and therefore less valuable. The “origin of color” of a colored diamond (that is, whether its color is natural or laboratory-treated) is disclosed on a GIA Grading Report."
To read more of the article, "The World's Fascination with Fancy Colored Diamonds" by GIA, CLICK HERE.