A tight correlation between colour and magnitude is known to exist both for galaxies in clusters at nearby redshifts, and those at high redshifts. I will present the colour-magnitude relations of a massive, X-ray luminous cluster of galaxies at z=0.9 and examine its evolution compared to models of synthetic stellar populations and the properties of the Coma cluster. It is found that while the majority of the galaxies have old stellar populations (z_f > 2), there is some evidence of a younger, bluer stellar population. The origin of this young component will be discussed. Mergers between galaxies are found to be unimportant in the evolution of the colur-magnitude relation.