Large inclusions, or many of them, that slightly diminish the brilliance of the stone to the naked eye. Enough inclusions that the brilliance of the stone is noticeably diminished. Although many diamonds may appear colorless to the untrained eye, the majority contain very slight traces of yellow or brown. Stones with colors of D,E, or F are considered colorless and make for a very bright stone. Farther down the color scale, diamonds have an increasing yellow or gray tint that makes the stone a bit darker.
Colorless stones are expensive.
The color difference only really shows when compared side by side with a different stone. More often than not, the color of stones at retail establishments is very inaccurate. Unless you are an expert, you MUST get a certified stone to make sure that you are getting what you think you are getting. Yellow Very Light Yellow Light Yellow Color quality is critical because a diamond's value increases dramatically the more colorless it is. Color determination consists of comparison with master set of diamonds under daylight conditions.