  (Abridged) We investigate the properties of galaxies between the blue and the
red sequence (i.e., the transition region) by combining UV and NIR imaging to
HI line observations for a volume-limited sample of nearby galaxies. We confirm
the existence of a tight relation between colour and HI-fraction across all the
range of colours, although outside the blue cloud this trend becomes gradually
weaker. Transition galaxies are divided into two different families, according
to their atomic hydrogen content. HI-deficient galaxies are the majority of
transition galaxies in our sample. They are found in high density environments
and all their properties are consistent with a quenching of the star formation
via gas stripping. However, while the migration from the blue cloud is
relatively quick (i.e., <=1 Gyr), a longer amount of time (a few Gyr at least)
seems required to completely suppress the star formation and reach the red
sequence. At all masses, migrating HI-deficient galaxies are mainly disks,
implying that the mechanism responsible for today's migration in clusters
cannot have played a significant role in the creation of the red sequence at
high-redshift. Conversely, HI-normal transition galaxies are a more
heterogeneous population. A fraction of these objects show evidence for
accretion/minor-merging events suggesting that at least part of the HI
reservoir has an external origin. The detailed evolution of such systems is
still unclear, but our analysis suggests that some galaxies might have migrated
back from the red sequence after accretion events. Our study clearly shows the
variety of evolutionary paths leading to the transition region and suggests
that the transition galaxies may not be always associated with systems quickly
migrating from the blue to the red sequence.
