Most diamonds range in colour from colourless to slightly yellow. The colour grade is determined by comparison with a series of master stones, the best grade being colourless - a perfect diamond has no colour at all (D Colour: Exceptional White +). Diamonds can also occur in all ranges of colours, from brownish to striking yellow, pink, purple, red, blue and green. These are called 'fancy colours'. Some diamonds react to UV-light (black light) with a grading scale for fluorescence, which ranges from 'nil', 'slight' or 'medium' to 'strong'.