Most gem quality diamonds fall within a range from “D” colourless to “Z” light yellow. Almost all have a trace of yellow, brown or gray body colour. The colour of a diamond is determined on the basis of a visual assessment by comparing it to a set of master diamonds under laboratory conditions. The differences between colours are very subtle and graded according to the International Colour Grading Scale. With the exception of some natural fancy colours, such as blue, pink, purple, yellow or red, the colorless grade is the most valuable.
Due to inaccuracy and inconsistency, colour machines are not used to grade diamonds by any of the internationally recognised laboratories. The prescribed method for grading colour is by comparing the diamond to a registered master set.