You can add color to ModPodge using watercolor or other paint mediums. It will dry shiny and semi-transluscent. Acrylic modeling paste might be a better solution, if you want them to look like creamy frosting. SOunds like a fun project!
> I am planning to do a lesson on Wayne Thiebaud "Paint a cake". It
> will bedone on drawing paper with cheap thin acrylic paint. Is there a way to
> makethe paint thicker to add frosting flowers?