As you are likely aware a true colorless diamond is quite rare. Most diamonds have a slight tint of yellow. In most case we don’t see the color until the tint of yellow becomes pronounced. Each of us see color differently. What appears to be a diamond with a slight tint of yellow for one will appear colorless for another. Diamonds mounted are more difficult to grade for color. The contrast between the yellow color of a 14 K yellow gold or 18 K yellow mounting and a diamond tends to mask the diamond’s color.
A diamond is formed from carbon. Trace amounts of nitrogen combined with carbon in the crystal color the diamond yellow. These impurities may be as small as one part per million. Imperfections in the crystal lattice can also alter the diamond’s color. Deviations in the crystal lattice are believed to be caused when intense pressure is applied during the diamond forming process.