Distribution of the elliptically averaged observed (grey regions) and modelled (solid lines) colour indices for the UBVRI filters along the major (four top panels) and minor (bottom panel) semi axis. The scatter of the grey region indicates the uncertainty of observations, ellipse fitting and conversion to the UBVRI system. Points with error-bars are observational data as compiled in Tempel et al. (2010), shown for comparison; the I data were not available for the major axis.