  We present evolutionary synthesis models for galaxies of spectral types Sa
through Sd with starbursts of various strengths triggered at various redshifts
and study their photometric evolution before, during, and after their bursts in
a cosmological context. We find that bursts at high redshift, even very strong
ones, only cause a small blueing of their intrinsically blue young parent
galaxies. At lower redshift, even small bursts cause a significant blueing of
their intrinsically redder galaxies. While the burst phase is generally short,
typically a few hundred Myr in normal-mass galaxies, the postburst stage with
its red colors and, in particular the very red ones for early bursts at high
redshift, is much longer, of the order of several Gyr. Even without any dust,
which in the postburst stage is not expected to play an important role anyway,
models easily reach the colors of EROs in the redshift range z=2 through z=0.5
after starbursts at redshifts between 2 and 4. We therefore propose a third
alternative for the ERO galaxies beyond the two established ones of passive
galaxies vs. dusty starbursts: the dust-free post-(strong-)starbursts. A very
first comparison of our models to HDF data with photometric redshifts shows
that almost all of the outliers that could not be described with our chemically
consistent models for undisturbed normal galaxy types E through Sd can now be
explained very well. Galaxies in the redshift range from z=2.5 to z=0.5 that
are redder, and in some cases much redder, than our reddest undisturbed model
for a high-metallicity classical elliptical are well described by
post-starburst models after starbursts at redshifts between 2 and 4. Galaxies
bluer than our bluest low metallicity Sd model, most of which have redshifts
lower than 1, are well explained by ongoing starbursts.
