In regard to the color of a diamond, less is more.
Diamonds are color graded on a scale defined by GIA (Gemological Institute of America) on an alphabetical scale starting at D color or colorless, up to Z color or light yellow or brown, where value is based on how little color a diamond exhibits. (An exception for fancy colored diamonds, such as blue and pink, exists where stronger color is more valuable.)
Diamond color is graded in controlled environments under specific lighting and by comparing diamonds to stones of know color grades. Although each diamond color grade is specifically defined, the variations appear very subtle to the untrained eye, while the effect of color can be more than subtle on the value of a diamond.