  We have determined a dust-free colour-magnitude (CM) relation for spiral
galaxies, by using I-K colours in edge-on galaxies above the plane. We find
that the scatter in this relation is small and approximately as large as can be
explained by observational uncertainties. The slope of the near-IR CM relation
is steeper for spirals than for elliptical galaxies. We suggest two possible
explanations. First, the difference could be caused by vertical colour
gradients in spiral galaxies. In that case these gradients should be similar
for all galaxies, on average 0.15 dex in [Fe/H] per scale height, and increase
for later galaxy types. The most likely explanation, however, is that spirals
and ellipticals have intrinsically different CM relations. This means that the
stars in spirals are younger than those in ellipticals. The age, however, or
the fraction of young stars in spiral galaxies, would be determined solely by
the galaxy's luminosity, and not by its environment.
