The ‘colours’ are sometimes referred to as ‘red’, ‘white’, and ‘blue’, which strikes me as being both confusing (since I do not think of white as a colour) and revealing of some sort of misplaced patriotism. Sometimes they are called ‘red’, ‘green’, and ‘blue’, which is better; but since the association between ‘quark colour’ and the colour receptors in the eye has in any case no scientific justification, I shall use ‘red’ (R), ‘yellow’ (Y), and ‘blue’ (B) instead. This choice of terminology has the advantage that I can ‘mix’ my colours more easily, and we note that ‘orange’, ‘green’, and ‘purple’ (these being regarded as the quantum superpositions of the original R, Y, and B) would do just as well as the original set.
(Roger Penrose, The Road to Reality, 25.7)