  In colour-magnitude diagrams most galaxies fall in either the ``blue cloud''
or the ``red sequence'', with the red sequence extending to significantly
brighter magnitudes than the blue cloud. The bright-end of the red sequence
comprises elliptical galaxies (Es) with boxy isophotes and luminosity profiles
with shallow central cores, while fainter Es have disky isophotes and power-law
inner surface-brightness (SB) profiles. An analysis of published data reveals
that the centres of galaxies with power-law central SB profiles have younger
stellar populations than the centres of cored galaxies. We argue that thermal
evaporation of cold gas by virial-temperature gas plays an important role in
determining these phenomena. In less massive galaxies, thermal evaporation is
not very efficient, so significant amounts of cold gas can reach the galaxy
centre and fill a central core with newly formed stars, consistent with the
young stellar ages of the cusps of Es with power-law SB profiles. In more
massive galaxies, cold gas is evaporated within a dynamical time, so star
formation is inhibited, and a core in the stellar density profile produced by
dissipationless dynamics cannot be refilled. The different observed properties
of AGN in higher-mass and lower-mass ellipticals are also explained because in
the former the central black holes invariably accrete hot gas, while in the
latter they typically accrete cold gas. An important consequence of our results
is that at the present time there cannot be blue, star-forming galaxies in the
most massive galactic halos, consistent with the observed truncation of the
blue cloud at L*. [abridged]
