  The correlation between [Mg/Fe] and galaxy mass found in elliptical galaxies
sets a strong constraint on the duration of star formation. Furthermore, the
colour-magnitude relation restricts the range of ages and metallicities of the
stellar populations. We combine these two constraints with a model of star
formation and chemical enrichment including infall and outflow of gas to find
that the trend towards supersolar [Mg/Fe] in massive ellipticals excludes a
pure metallicity sequence as an explanation of the colour-magnitude relation.
An age spread is required, attributable either to a range of star formation
efficiencies or to a range of infall timescales. We find that the inferred
range of stellar ages is compatible with the small scatter and the redshift
evolution of the colour-magnitude relation. Two alternative scenarios can
explain the data: fixed infall with an efficiency which is linearly dependent
on mass, or a fixed efficiency with a mass-dependent infall timescale
t~1/sqrt(M). We conclude that the actual scenario may well involve a
combination of these two parameters, with mass dependencies which should span
the range of those given above.
