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To properly understand the evolution of high-redshift galaxy clusters, bothpassive and star-forming galaxies have to be considered. Here we study theclustering environment of 21 radio galaxies and quasars at 1<z<2.5 from thethird Cambridge catalog (3C). We use optical and near-infrared Hubble SpaceTelescope images with a 2' field-of-view, where the filters encompass therest-frame 4000 Angstroem break. Passive red and star-forming blue galaxieswere separated in the color--magnitude diagram using a redshift-dependent cutderived from galaxy evolution models. We find that about 16 of 21 radio sourcesinhabit a galaxy overdensity on scales of 250 kpc (30") projected radius. Thesample shows a diversity of red and blue overdensities and also sometimes adeficiency of blue galaxies in the center. The following tentative evolutionarytrends are seen: extended proto-clusters with only weak overdensities at z >1.6, red overdensities at 1.2<z<1.6, and red overdensities with an increaseddeficit of central blue galaxies at z<1.2. Only a few 3C sources show a blueoverdensity tracing active star-formation in the cluster centers; this raritycould indicate that the powerful quasar activity may quench star-formation inthe vicinity of most radio sources. The derived number of central luminous redgalaxies and the radial density profiles are comparable to those found in localclusters, indicating that some 3C clusters are already mass-rich and compact.

Clustering of red and blue galaxies around high-redshift 3C radio sources as seen by the Hubble Space Telescope