  ABRIDGED- The UV-optical color magnitude diagram (CMD) of rich galaxy groups
is characterised by a well developed Red Sequence (RS), a Blue Cloud (BC) and
the so-called Green Valley (GV). Loose, less evolved groups of galaxies likely
not virialized yet may lack a well defined RS. This is actually explained in
the framework of galaxy evolution. We are focussing on understanding galaxy
migration towards the RS, checking for signatures of such a transition in their
photometric and morphological properties. We report on the UV properties of a
sample of ETGs galaxies inhabiting the RS. The analysis of their structures, as
derived by fitting a Sersic law to their UV luminosity profiles, suggests the
presence of an underlying disk. This is the hallmark of dissipation processes
that still must have a role in the evolution of this class of galaxies. SPH
simulations with chemo-photometric implementations able to match the global
properties of our targets are used to derive their evolutionary paths through
UV-optical CDM, providing some fundamental information such as the crossing
time through the GV, which depends on their luminosity. The transition from the
BC to the RS takes several Gyrs, being about 3-5 Gyr for the the brightest
galaxies and more long for fainter ones, if it occurs. The photometric study of
nearby galaxy structures in UV is seriously hampered by either the limited FoV
of the cameras (e.g in HST) or by the low spatial resolution of the images (e.g
in the GALEX). Current missions equipped with telescopes and cameras sensitive
to UV wavelengths, such as Swift-UVOT and Astrosat-UVIT, provide a relatively
large FoV and better resolution than the GALEX. More powerful UV instruments
(size, resolution and FoV) are obviously bound to yield fundamental advances in
the accuracy and depth of the surface photometry and in the characterisation of
the galaxy environment.
