Taken from Fig. 3 of
that paper, the figure shows the "g-i color versus i-band absolute magnitude
relation of all galaxies in the C[oma]S[upercluster] coded according to Hubble type: red = early-
type galaxies (dE-E-S0-S0a); blue = disk galaxies (Sbc-Im-BCD); green = bulge
galaxies (Sa-Sb)... Contours of equal
density are given. The continuum line g-i =
-0.0585 *(Mi + 16) + 0.78 represents the empirical separation between the
red-sequence and the remaining galaxies.
The dashed line illustrates the effect of the limiting magnitude r=17.77 of the
spectroscopic SDSS database, combined with the
color of the faintest E galaxies g-i ~0.70 mag.."
Be sure you understand this diagram and what it tells us about galaxies.
Figure 3 from Gavazzi
et al (2010)