  We use the $C_{2}4668$, $Fe4383$, $H\gamma_A$ and $H\delta_A$ spectral
absorption line indices, together with U- and V-band photometry of 101 galaxies
in the Coma cluster, to investigate how mean age and metal abundance correlate
with galaxy luminosity. In particular, we use the line index measurements to
address the origin of the colour-magnitude relation (CMR). We find that the CMR
in Coma is driven primarily by a luminosity-metallicity correlation. We
additionally show evidence for a relation between age and luminosity, in the
direction predicted by the semi-analytic hierarchical clustering models of
Kauffmann & Charlot, but this is only present in the $C_24668$ index models,
and could be an effect of the lack of non solar abundance ratios in the Worthey
models used.
  By comparing deviations from the CMR, with deviations in absorption index
from analogous `index-magnitude' relations, we find that colour deviations
bluewards of the mean relation are strongly correlated with the hydrogen Balmer
line series absorption. We show that the properties of these blue galaxies are
consistent with the presence of a young stellar population in the galaxies,
rather than with a reduced metallicity.
