  The distribution of galaxies has been studied to show the difference between
the blue cloud and red sequence and to define the green valley region. However,
there are still many open questions regarding the importance of the green
valley for understanding the morphological transformation and evolution of
galaxies, how galaxies change from late-type to early-type and the role of AGN
in galaxy formation and evolution scenario. The work focused on studying in
more details the properties of green valley galaxies by testing the six most
used selection criteria, differences between them, and how they may affect the
main results and conclusions. The main findings are that, by selecting the
green valley galaxies using different criteria, we are selecting different
types of galaxies in terms of their stellar masses, sSFR, SFR, spectroscopic
classification and morphological properties, where the difference was more
significant for colour criteria than for sSFR and SFR vs. stellar-mass
criteria.
