Getting color to stick to plastic bags is very dependent on the paint.
I would test spray enamel on one and see if it binds or if it melts.
If it binds then, I’d make reverse stencils out of masking or painters tape.
I don't know about bags but my husband makes some darn cute things out of cereal boxes. He makes models of ideas he has for bigger projects.
I think this is what you're looking for! Hope it helps!
You could decorate with stickers and tie with ribbon for a simple bag.