A great description and explanation of fancy color diamonds from gia.edu
In diamonds, rarity equals value. With diamonds in the normal range, value is based on the absence of color, because colorless diamonds are the rarest. With fancy color diamonds—the ones outside the normal color range—the rarest and most valuable colors are saturated pinks, blues, and greens. In all cases, even very slight color differences can have a big impact on value.
Compared to fancy yellows and browns, diamonds with a noticeable hint of any other hue are considerably more rare. Even in light tones and weak saturation, as long as they show color in the face-up position, they qualify as fancy colors. Red, green, and blue diamonds with medium to dark tones and moderate saturations are extremely rare.
Grading fancy color diamonds is complex and specialized, and it takes highly trained laboratory graders to complete the process accurately.
The GIA system for color-grading fancy color diamonds is designed to accommodate the fact that not all colored diamonds have the same depth of color. For example, yellow diamonds occur in a wide range of saturations, while blue diamonds do not.
Diamonds with red or reddish colors are extremely rare and highly valued. Pure pinks are more popular than diamonds that are purplish, orangy, brownish, or grayish. Trade professionals market some very attractive stones in this category as “rose-colored,” and some stones with purplish tints as “mauve” diamonds.
Blue diamonds are extremely rare. They generally have a slight hint of gray, so they’re rarely as highly saturated as blue sapphires. Their color is caused by the presence of boron impurities—the more boron, the deeper the blue.