Color-Magnitude relation for sdss galaxies. Note that for this graph the numbers of low luminosity galaxies have been boosted (by Vmax / V) to correct for the fact that the SDSS is a flux limited sample and so misses low luminosity galaxies at greater distances. From Blanton et al 2003 ApJ 594 186 o-link
Several bi-variate density plots, all corrected for Malmquist bias (increasing the lower luminosity galaxies because they are underrepresented). Notice the color distribution (at least in g-r) is clearly bi-modal. You can see a summary of this work on Mike Blanton's web page: o-link