a) NUV-r histogram of our HCG galaxy sample shown as a black solid line. The corresponding histograms of the early- and late-type galaxies found in dynamically “young” groups are shown with the red and blue shaded areas, respectively. b) Same as in a), but for the galaxies in dynamically “old” groups. The region of 3  <  NUV-r  <  5, identified as “green valley”, is marked with the vertical dashed lines.