Colour-magnitude diagram of 9737 sources from the UKIDSS-GPS within the WSRT mosaic (see Sect. 2.2). The reddening curve computed from redshift z = 0.02 to z = 0.4 is shown by the black line with open circles representing the redshift bin and the Galactic dust reddening vector for a K-band extinction of 0.56 mag is indicated with the arrow. Dashed lines represent the boundaries of red sequence cluster members compared to blue outliers (vertical line) or background galaxies (slanted line).