Colour–magnitude correlations for (R − I) versus I, (I − z′) versus I, and (J − K_s) versus K_s, from top to bottom, respectively. An extinction vector for A_v = 1 is plotted in all cases, indicating that any variation due to variable extinction by dust particles similar to those found in the interstellar medium would cause a quite different colour magnitude correlation. The dotted lines show the least-squares fit to the data, which is not statistically significant in any of the three correlations.