  We investigate the [3.6] - [4.5] Spitzer-IRAC colour behaviour of the
early-type galaxies of the SAURON survey, a representative sample of 48 nearby
ellipticals and lenticulars. We investigate how this colour, which is
unaffected by dust extinction, can be used to constrain the stellar populations
in these galaxies.
  We find a tight relation between the [3.6]-[4.5] colour and effective
velocity dispersion, a good mass-indicator in early-type galaxies. Contrary to
other colours in the optical and near-infrared, we find that the colours become
bluer for larger galaxies. The relations are tighter when using the colour
inside r_e, rather than the much smaller r_e/8 aperture, due to the presence of
young populations in the central regions. We also obtain strong correlations
between the [3.6]-[4.5] colour and 3 strong absorption lines (H beta, Mg b and
Fe 5015). Comparing our data with the models of Marigo et al., which show that
more metal rich galaxies are bluer, we can explain our results in a way
consistent with results from the optical, by stating that larger galaxies are
more metal rich. The blueing is caused by a strong CO absorption band, whose
line strength increases strongly with decreasing temperature and which covers a
considerable fraction of the 4.5 micron filter. In galaxies that contain a
compact radio source, the [3.6]-[4.5] colour is generally slightly redder than
in the other galaxies, indicating small amounts of either hot dust, non-thermal
emission, or young stars near the center.
  We find that the large majority of the galaxies show redder colours with
increasing radius. abbreviated...
