The vast majority of untrained observers (and many gemologists) cannot distinguish a color grade from the one just above or below unless the diamonds are compared side by side in a controlled environment. Even when side-by-side, changes in color are difficult to detect in I color and higher diamonds.
Color becomes much harder to detect once a diamond is set in a ring and placed in an environment that contains color (as opposed to the all white background used in color grading). For instance, an H color diamond may look as colorless as a D when set in a ring under normal lighting conditions, especially if the two are not compared side by side.
For the best value in what would appear to the naked eye as a colorless diamond, look for G-J diamonds. Because color is easier to detect in larger diamonds (just as a large carafe of white wine shows more color than a small glass), opt for G-H in diamonds over 1 carat, and I-J for those under 1 carat. Once set in a ring, these diamonds will look just like higher color grade diamonds. Instead of investing in higher color, invest in higher cut, the most important factor in a diamond’s brilliance.
Because diamonds with more facets reflect more light, they tend to hide color better than other shapes. So, consider round, princess or other modified brilliant cuts over step cuts such as emerald or asscher if you are concerned about color.
If you are concerned primarily about carat weight, and are on a tight budget, consider a yellow gold setting and a brilliant cut diamond in the K-L color range. The yellow gold will complement the faint body color of the diamond.
If, while shopping for a diamond, you are ever given a color range (e.g. G-H) as opposed to a specific grade, the diamond is not certified by GIA. The seller is only estimating the diamond’s color using GIA terminology.
Colorless (D-F) fluorescent diamonds sell at a 5-15% discount to non-fluorescent diamonds since the fluorescence is perceived as a defect. In fact, the visible effects of faint to medium fluorescence are perceptible only to a gemologist using a special UV light source.
Because the fluorescent glow is usually blue (which is the complementary color to yellow) fluorescence can make diamonds of I-M color appear up to one grade whiter. For this reason, I-M diamonds tend to sell at a slight premium when they possess Medium to Very Strong fluorescence.
Overall, fluorescence should not be a major factor in the diamond purchase since its effects on appearance are negligible, if not slightly positive. The exception would be to exercise caution in purchasing a diamond with strong or very strong fluorescence in D-H color diamonds (which do not possess enough yellow color to offset the blue fluorescence).