How colours are used for description in Sir Gawain and the Green Knight is a crucial question, especially with respect to green, whose significance and polysemy have been much commented on. Yet, all the basic colours of the Western medieval world are to be found in this Arthurian romance in verse, and they contribute to making the text and the images it evokes more vivid and colourful. My purpose here is to study the use of colours in SGGK, drawing on the life-long work of the historian of colours Michel Pastoureau, as well as on some simple linguistic notions, in order to analyse how colours are combined within a broader, culturally-constructed, conceptual system.