  We use our integrated SDSS photometry for 96 globular clusters in $g$ and
$z$, as well as $r$ and $i$ photometry for a subset of 56 clusters, to derive
the integrated colour-metallicity relation (CMR) for Galactic globular
clusters. We compare this relation to previous work, including extragalactic
clusters, and examine the influence of age, present-day mass function
variations, structural parameters and the morphology of the horizontal branch
on the relation. Moreover, we scrutinise the scatter introduced by foreground
extinction (including differential reddening) and show that the scatter in the
colour-metallicity relation can be significantly reduced combining two
reddening laws from the literature. In all CMRs we find some low-reddening
young GCs that are offset to the CMR. Most of these outliers are associated
with the Sagittarius system. Simulations show that this is due less to age than
to a different enrichment history. Finally, we introduce colour-metallicity
relations based on the infrared Calcium triplet, which are clearly non-linear
when compared to $(g^\prime-i^\prime)$ and $(g^\prime-z^\prime)$ colours.
