I had a wardrobe mistress with a brilliant series of solutions for this - will try to find her notes - cuz we needed foam that wouldn't stain either (for a different show) but our problem was that the actor needed to ingest as well as get costumes dirty with the foam. In the meantime, since I don't recall this needing to be edible in Bullshot, try whipping soapy water/soft soap into a froth and use that - and clearly it won't stain since it is soap, after all.