Fancy color diamonds gain their value from their rarity. Yellow and brown diamonds certainly have their appeal, but they're also the most common diamond color. A diamond that shows other hues becomes far more rare. Red is the rarest diamond color, but others are also scarce - and valuable. Here's what you should look for in these diamonds.
The first step is to understand rarity a bit better:
Red diamonds are the rarest diamond color. They have further subsets depending on other colors that may be present, such as rose or mauve.
Blue diamonds are next rarest. They get their color from the presence of boron impurities. The more there are, the bluer the diamond. Blue diamonds often have shades of gray, which are influenced by clarity and cut.
Green diamonds are generally lighter in color. They don't come in forest or emerald greens, they come in yellow and pale greens. The nature of green diamonds mean that any cut should leave a much of the original rough as possible. The green hue is typically only present at the surface.
Brown and yellow (or canary) diamonds are more common, yet still very attractive.
You can also find fancy white, black, or gray diamonds. The white refers to milky white diamonds, not clear diamonds.
Your next concern is the clarity of the diamond. This isn't always as important for fancy color diamonds as it is for the more common varieties. Rarity of color can overcome clarity issues if the color still shows in an extraordinary way. Clarity is still preferred, of course.
With normal white diamonds, a cut with broader facets can make one flash, and a cut with many, smaller facets can make it glitter. Shape similarly influences the appearance and brilliance of a diamond. The same thing applies to fancy color diamonds, but with the added presence of how color shows through these cuts and the resulting facets. Different cuts can make the color seem deeper in color and more absorbing, or broader and more brilliant.
Fancy color diamonds are graded on all these and more elements. Understand that quality of color doesn't just work on a 2-D light-to-intense scale. Obviously, most people prefer fancy intense to fancy light because the color quality is much more apparent. The scale is more three-dimensional than just that, with important qualifications such as “fancy vivid” that differ greatly from “fancy dark” and “fancy deep.”
Also make sure that the diamonds in question are natural and not treated. Natural color diamonds have produced their brilliant, complex colors over a billion years. Treated color diamonds look more like fruit juice colors.
Above all, ask questions and learn until you understand. Fancy color diamonds are exceptional choices, and it's worth it to learn what exactly makes the color and quality of the one that appeals to you so special. They're each unique, and have their own story.