Many people would be surprised to hear that diamonds have any colour at all, but in fact they can vary widely. Most diamonds have a subtle tint of yellow to a varying degree, but there can also be very rare stones that have other pure colours, such as yellow, brown, red, green, pink or blue.

The less yellow there is in a stone, the greater is its desirability and therefore its worth. However, in normal situations most lay people would probably be unable to tell the difference between different grades of colour in diamonds.

Grading stones based on their colour or its absence is a highly skilled task, and is done under controlled lighting conditions. The stone to be tested may be compared against a reference stone whose officially graded colour is known.

In describing an exceptionally fine diamond, authors often use the expression "blue white" to praise its colour. It is important to be aware that this expression does not in fact relate to the presence of a blue tint in the stone. The best stones are completely without colour, but a blue quality may result from the phenomenon of fluorescence. In exceptional diamonds, the fluorescence may be visible even in daylight, and this is what led to the phrase "blue white".

Colour grading scale from totally colourless to light yellow. The differences between one grade and another are very subtle, as can be seen by the number of grades within any one category.

How colour is defined in diamonds in two international colour grading systems: