Diamond grading standards fall into four distinct categories, the "Four C's":
Think of these as sliding scales
Grading A Diamond's Color
Generally, the less color a diamond shows, the more valuable it is, given it's clarity. There are exceptions, like extremely valuable "natural fancy colored diamonds" and less expensive "color enhanced" stones, but I will focus here on standard diamonds. This chart would indicate the GIA, AGS, and other European grading systems, along with older terminology sometimes still mentioned, but not used by todays labs... Using GIA(Gemological Institute of America) terminology as the standard, "D" to "F" are considered "colorless", or the finest white. The next level down, "G" to "J" are considered "near colorless". The majority of diamonds used in jewelry sold commercially are in this color range because they appear colorless to the untrained eye.
Across the scale, from a perfect colorless D to a markedly colored Z, considering that the group of upper grades all appear colorless to most, I suggest that a good value on the sliding scale of color is at the"G/H" range for the best "look" in a white diamond, at the "best price".... While subtle graduations do exist, and are measurable by a gemologist, to all but the trained eye, why pay for something you can't possibly detect? You might even save a week's salary or two! :)
Diamonds graded from K on down through Z show visibly more brownish or yellowish color until it gets concentrated enough to be considered "fancy" color. Some people actually prefer the "warmer" tones of diamonds in the K-M grades. It doesn't matter the color of the diamond, if it's a well cut stone, it will still be beautiful and radiate brilliance.