Caption: A cartoon galaxy color-magnitude diagram.
Galaxy color-magnitude diagrams are sort of the galaxy analogues to Hertzsprung-Russell (HR) diagrams for stars.
Features:
A color index is the bluer minus the redder of the pair of passband filter magnitudes and bigger the difference, the redder the color index for the astronomical object. The color index is probably B-V.
But note that in absolute true color galaxies look white???. "Blue" galaxies have more blue light relative to light red than "red" galaxies, but that is NOT the human psychophysical response in their absolute true color.
See Extended Features below for more discussion of the color of galaxies.
Between the two major populations it has been suggested that there is a relatively unpopulated green valley which is just a whimsical name since the galaxies in the green valley do NOT look green in any true-color sense. They probably look a mixture of the other two populations in true-color.
There are green bean galaxies and green pea galaxies, but they only look greenish in certain standard imaging passband filters, probably NOT in true-color.
The red sequence galaxies are quenched galaxies: i.e., those for which star formation rate (SFR) is less than ∼ 10 % of SFR in galaxies recognized as star forming galaxies (mostly spiral galaxies and irregular galaxies) and maybe star formation is turned off completely (see, e.g., Cimatti 2020, p. 54). As a shorthand, we usually say quenched galaxies have NO star formation without qualification or star formation turned off without qualification.
Similarly, quenched galaxies look white hot or somewhat yellow-white in absolute true color particularly as their stellar effective temperatures are n the white hot range ∼ 1500--11000 K see Temperature of a "White Hot" Object, Carine Fang, 2001, bottom of page; blackbody_spectra.html). Thousands of gigayears in the future (wildly extrapolating to the Λ-CDM model cosmic future), the quenched galaxies will become redder in the photometric sense (i.e., in B-V color index) since the only stars left doing nuclear burning will be red dwarfs which have lifetimes up to ∼ 10**4 gigayears (see Wikipedia: Red dwarf: Description and characteristics). However, the quenched galaxies will still probably look white hot in true color (as seen in space) because their stellar effective temperatures are ∼ > 2300 K which is still in the white hot range.