Distribution of galaxies with  passing our size selection in g − z vs z − K_s colour space. The black line indicates the colour selection z − K_s > min[g − z, 2.5] employed in our analysis. The left panel shows a random 50% fraction of the galaxies in the CANDELS/COSMOS 3D-HST grism area, with colours and symbols indicating various ranges in the best redshift estimate from 3D-HST. The right panel shows the galaxies passing the shape selection in our catalogue for RCS2 J2327. The excess of galaxies around g − z ≃ 3 and z − K_s ≃ 1 corresponds to the cluster red sequence, which is efficiently removed from our background sample, along with bluer cluster members located near g − z ≃ 1.2 and z − K_s ≃ 0.3.