I have a plastic-bristled paintbrush I keep with my kitchen tools, just for food coloring painting! There's also a bento tool ($1.50 at Daiso stores, or $3ish from bento suppliers like allthingsforsale.com) that's a fillable writing tube for painting with mayo, soy sauce, etc. Or use ketchup/mustard/etc and snip a teeeeeeny corner off a plastic baggie like with frosting bags. Or I'm also just lazy and find cheap frosting tubes in a variety of colors (often in little kits on sale after holidays!) and use those!