If the plastic soldiers are made of polyethylene or polypropylene plastics, the paint will likely not adhere to the surface. Both polyethylene and polypropylene are materials with very low surface energy; fluid do not "wet" the surface - kind of like how water beads on the hood of a newly waxed car. High surface energy materials have better adhesion to glues, tapes and paints. I use a polyethylene tray for mixing my paints at home for this reason, paint does not adhere to it and it is very easy to clean out after the fact.

Jmac