Colour–magnitude correlations for K_s − 4.5   μm and K_s − 3.6   μm versus K_s. The lines show the linear fit to each correlation. In both cases the correlations are significant at a 3σ level. Extinction vectors corresponding to A_v = 1 are shown for comparison. The positive slope in both correlations agrees with variability caused by variations in extinction by dust particles smaller than the observing wavelength.