  Current models of galaxy formation predict satellite galaxies in groups and
clusters that are redder than observed. We investigate the effect on the
colours of satellite galaxies produced by the ram pressure stripping of their
hot gaseous atmospheres as the satellites orbit within their parent halo. We
incorporate a model of the stripping process based on detailed hydrodynamic
simulations within the Durham semi-analytic model of galaxy formation. The
simulations show that the environment in groups and clusters is less aggressive
than previously assumed. The main uncertainty in the model is the treatment of
gas expelled by supernovae. With reasonable assumptions for the stripping of
this material, we find that satellite galaxies are able to retain a significant
fraction of their hot gas for several Gigayears, thereby replenishing their
reservoirs of cold, star forming gas and remaining blue for a relatively long
period of time. A bimodal distribution of galaxy colours, similar to that
observed in SDSS data, is established and the colours of the satellite galaxies
are in good agreement with the data. In addition, our model naturally accounts
for the observed dependence of satellite colours on environment, from small
groups to high mass clusters.
