The best color for a diamond is no color or for the diamond to be colorless. Diamonds allow light to be reflected and dispersed as a rainbow of color. This light dispersion, or color flash, has no effect on the technical grading of color.
The absolute finest colorless stone carries a D rating, descending through each letter of the alphabet to Z. The grade of Z designates a diamond’s color of light yellow, brown, or gray. As the body color becomes more intense, the grade for color descends the scale. These gradations are so slight and precise that discerning a single grade (even by an expert) under less than ideal laboratory conditions is extremely difficult.
Unless you are looking for a “fancy-colored” diamond, one should stick with the D–K colored diamonds and avoid L–Z colored diamonds.
Diamonds below H color with fluorescence may actually appear whiter in a light with a UV component. This is because blue is complimentary to yellow and will, consequently, enhance the value of the diamond.
Fluorescence is an identifying characteristic of a natural diamond. If a diamond has fluorescence, it displays a tint of blue when exposed to ultraviolet light. It is one of the tests that diamonds undergo in laboratory conditions when being independently graded by labs such as the GIA. It is estimated that around 50% of all diamonds have some degree of fluorescence. The GIA has 5 grades for fluorescence: None, Faint, Medium, Strong, and Very Strong. Please bear in mind, however, that these images are taken under laboratory conditions and intense UV light.
The purest and rarest diamonds are virtually colorless. Although many diamonds may appear colorless to the untrained eye, the majority contain very slight traces of yellow, brown, or gray. Color quality is critical. A stone’s beauty and value usually increase dramatically the more colorless it is.