Though I haven't used it on rubber, I know leather paint can crack on stress points. We paint black riding boots white for stormtrooper boots, and they commonly crack across the toe line where the boot flexes the most.
So using that as an assumption, I don't think it would work the best on rubber.
You can pick up vinyl/upholstery paints from a lot of car stores, which you may have more luck with. Or even hit up a paint store (not a hardware store where they don't know what they're talking about), and see what they suggest.
As with anything, always start with light coats, and build up the layers slowly with plenty of drying time in between.
I loaded my boots with white leather paint, about four coats within an hour and there's a very obvious crack.