  We present deep SofI and ISAAC near-infrared imaging data of the X-ray
luminous galaxy cluster RDCS J1252.9-2927. The ISAAC data were taken at the ESO
Very Large Telescope under very good seeing conditions and reach limiting Vega
magnitudes of 25.6 and 24.1 in the J- and Ks-bands respectively. The image
quality is 0.45 arc seconds in both passbands. We use these data to construct a
colour-magnitude (C-M) diagram of galaxies that are within 20 arc seconds of
the cluster center and brighter than Ks=24, which is five magnitudes fainter
than the apparent magnitude of a L* galaxy in this cluster. The C-M relation is
clearly identified as an over-density of galaxies with colours near J-Ks=1.85.
The slope of the relation is -0.05 and the intrinsic scatter is 0.06
magnitudes. Both the slope and the scatter are consistent with the values
measured for clusters at lower redshifts. These quantities have not evolved
from z=0 to z=1.24. However, significant evolution in the mean J-Ks colour is
detected. On average, the galaxies in RDCS J1252.9-2927 are 0.25 magnitudes
bluer than early-type galaxies in the Coma cluster. Using instantaneous
single-burst solar-metallicity models, the average age of galaxies in the
center of RDCS J1252.9-2927 is 2.7 Gyrs.
