Is it the kind of plasticine that you can bake/dry out to form a solid piece?
If it is soft plasticine, the acrylic paint will struggle to dry, and when/if it does it could crack and crumble with any pressure as the soft surface below would shift.
If you're desperate, and your acrylic is really thick, you could probably get away with putting a really thick layer on and allowing it to dry for ages, creating a kind of flexible plastic shell around your plasticine. The results would vary though, depending on the paint.
I would recommend trying to colour the plasticine with some food colouring or water soluble ink/dye. This can be messy though!
