Montage of many SDSS galaxies, showing a range of morphological type, inclination, and color
Right: Density plot of galaxy color (g-r) against galaxy luminosity (a reasonable proxy for mass). Left: the histogram of galaxy colors is approximately bimodal, suggesting galaxies either do or don't have ongoing or relatively recent star formation.
This famous plot identifies what have come to be known as the red and blue sequences, and the green valley (in between).