The green stripes on these bombs are painted on white decal film from Bare Metal. The paint is WEM Colourcoats RAF Light Green, thinned with lacquer thinner.
I've previously painted on Tauro's clear film also, but I couldn't find the sheet when I wanted it, so used the Bare Metal. The Tauro worked better.
Testors used to make sheets of solid color decal stock, but I wasn't able to find them again. Maybe they're discontinued. They still seem to produce a clear film and a white film for their decal system.
Tauro still makes sheets of solid-color decal film:
http://www.tauromodel.it/inglese/decacces.htm
but I don't know if they're available anymore in the US, or have to be ordered from Italia. Squadron used to handle them.
Amazon shows that they carry some:
https://www.amazon.com/Tauro-Decals/pages/11189478011
From experience with painting decal film, they do have a shelf life--how long, I'm not sure. I'd saved some painted film for a couple of years, but by then the paint flaked off when trying to apply them.
I usually paint the decal film, then use it within a day or so, before the paint gets too brittle.
Hope you have success with your project.