You can buy different colours of bark chippings. Depending on the situation they will dry out and lighten up anyway.
Eventually the bark chippings will rot down but the stones wouldn't. I don't think I would mix bark with stone but I suppose it's all a mater of individual taste. Whether they roll down or not would depend on how steep and the quantity you use.

Tricky. Bark at least sort of sticks a bit. Stone won't really. What about changing the look with pieces of tree stump, or planting some ferns and hostas in it if it's a shady slope?