Starfire's makeup would be just gold skin and green contacts, so that's not difficult; the expense would be showing her in flight and in action, and we saw a photo months ago of Anna Diop getting her face scanned for her digital stunt double. (I do wonder if they're going to try to recreate the George Perez "hair stretching back into a smoke trail" visual -- I was never quite clear on what that was supposed to represent physically.)
As for Beast Boy, creating various green digital animals might get a bit expensive, so they might have him change form only occasionally, or rely on certain recurring critters that they can reuse regularly. (Maybe they could borrow the digital models for Gorilla Grodd and Vixen's various ashe animals, and just change the skin and texturing.) Maybe they could use some live animals and paint them or digitally change their color.
Though I suppose they could drop the convention of having all Gar's animal forms be green. On the comics page, that's a useful device to connect his various forms in the readers' minds when the actual transformations take place between panels, but it may not be as necessary in live action.
