We derive deep I-band luminosity functions and colour-magnitude diagrams from Hubble Space Telescope imaging for eleven 0.2 < z < 0.6 clusters observed at various stages of merging, and a comparison sample of five more relaxed clusters at similar redshifts. The characteristic magnitude M* evolves passively out to z = 0.6, while the faint-end slope of the luminosity function is α ∼ −1 at all redshifts. Cluster galaxies must have been completely assembled down to M I ∼ −18 out to z = 0.6. We observe tight colour-magnitude relations over a luminosity range of up to 8 mag., consistent with the passive evolution of ancient stellar populations. This is found in all clusters, irrespective of their dynamical status (involved in a collision or not, or even within subclusters for the same object), and suggests that environment does not have a strong influence on galaxy properties. A red sequence luminosity function can be followed to the limits of our photometry: we see no evidence of a weakening of the red sequence to z = 0.6. The blue galaxy fraction rises with redshift, especially at fainter absolute magnitudes. We observe bright blue galaxies in clusters at z > 0.4 that are not encountered locally. Surface brightness selection effects preferentially influence the detectability of faint red galaxies, accounting for claims of evolution at the faint end.