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...
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