The figure below shows the color-magnitude distribution of galaxies
from the Sloan Digital Sky Survey at z<0.1.
The black solid and dashed contours represent the number density of
galaxies: logarithmically spaced with four contours per factor of ten.
The distribution is called bimodal because if a color histogram is plotted
there are two peaks corresponding to a red sequence
(generally early types) and a blue sequence (late types).
The red and blue shaded regions show the mean and dispersion
(plus/minus 1 sigma) of the red and blue sequences based on the fitting
described by Baldry et al. (2004).
The dotted lines represent galaxies of similar stellar masses
using the mass-to-light ratios of
the MPA Garching group
(Kauffmann et al. 2003).
The figure is a modified version of Figure 2 from
Baldry et al. (2004).
Link: download PS figure.
Ivan Baldry, written 2006 February 13.