The colours of early-type galaxies are tightly correlated with their
luminosities, the more luminous being redder. We investigated his
colour-magnitude relation using a large sample of 70000 E/S0 galaxies
from the SDSS, using different methods to determine the restframe g-r
colours. 
Some puzzling discrepancies were found, some which could be due to
subtle calibration problems. However, the marked difference in the
slopes of the colour-magnitude relations obtained from aperture and
half-light radius photometry seems to be caused by radial colour
gradients in the galaxies themselves.  
We investigate this further and find that the strength of colour
gradients in early-type galaxies depends significantly on a number of
properties including luminosity, radius and age. This may tell us
about their formation processes and subsequent merging. The colour
gradients of Brightest Cluster Galaxies are weaker and this may be a
consequence of multiple dry (non star forming) mergers.