[RallyRaider]

Depends on the opacity of the green topcoat, some greens can be fairly translucent. Modeller's green for instance is almost clear! Whereas many automotive laquers will cover over anything. If your green is in the former category I'd use a grey primer, then matt dark green, followed by the top coat. A white primer coat is probably not required, and may take too many coats to build up a satifactory dark shade. If you want a very deep, almost black finish, then a black base coat might even be worth trying.

Sorry no definitve answer. Experiment on some scrap with the paints you have and see what works best.