a) NUV-r histogram of the whole HCG galaxy sample is shown in black. The corresponding histograms of the early- and late-type galaxies of the sample, classified according to their optical morphology, are shown with the red and blue shaded areas. b) The NUV-r distribution of the galaxies hosting an AGN is indicated with the dashed dark line. As above, the early- and late-type hosts are indicated with the red and blue histograms. The region of 3  <  NUV-r  <  5, identified as “green valley”, is marked with the vertical dashed lines.