Rest-frame U − R colour (top), Dn4000 index (middle), and [O II] equivalent width (bottom) of our sample galaxies as a function of stellar mass. Galaxies in X-ray groups are shown with dark red stars, while galaxies in non-X-ray groups are shown with orange crosses and field galaxies with blue dots. The legend in the top panel indicates the total number of galaxies for each category. The coloured lines show the mean values in the stellar mass bins of the corresponding category of galaxies, with error bars indicating the 1σ error of the mean value.