  We select a sample of 70378 E/S0 (early-type) galaxies at 0<z<0.36 from the
Sloan Digital Sky Survey, excluding disk and star-forming galaxies. We estimate
g and r magnitudes in the observer- and rest-frames directly from the SDSS DR6
spectra; this provides an object-by-object estimate of the k-correction. We use
the k-corrections from the spectra to study the evolution of the rest-frame
colour-magnitude relation (CMR) and colour-sigma (velocity dispersion) relation
(CsigmaR). The evolution is very sensitive to the k-correction. Both the CMR
and CsigmaR evolve blueward with increasing redshift, approximately in
agreement with passive evolution models with age approx. 12 Gyr. The slope and
zero-point of the CMR depend on whether colours were defined in fixed physical
or angular apertures, a consequence of the fact that the centers of these
objects tend to be redder: the relation is steeper for fixed angular apertures.
One the other hand, the CsigmaR slope does not show this dependence on the
aperture in which the colour was defined, suggesting that colour gradients are
correlated with residuals from the sigma-M_r relation. As these residuals are
age indicators, our findings suggest that colour gradients correlate with the
age of the stellar population.
