This was introduced by 1ec2e08 which only called the color-generator
code when there *is* a ColorAttribute on the collection (with an
invalid color, which seems unlikely), rather than when the attribute
I thought this was the bug that made all new resources red (with caldav)
but actually that seems to come from the resource itself. So now I'm not
100% sure about this commit anymore, although it still seems to make
sense to me -- for other resources, possibly.