American tanks were painted olive drab, which is a green-brown colour that fades to brown, not to green. Thus, US Army tanks were rarely, if ever, green. Anything that says it’s US Army OD will be about the right shade; USAAF or USAF OD might be a bit off (usually too light) for a tank, but on the other hand, this might just compensate for the scale effect (that is, that you “need” to use a lighter shade the smaller the model is).
However, with “Sherman Firefly” you probably mean a Sherman with 17-pounder gun, right?
Those existed, but you can’t use a kit of a British one to make an accurate model, and they were never used in combat.
In that case you’ve got basically two choices: one is SCC 15 (Standard Camouflage Colour No. 15, these had no actual name as such) which is the British version of OD but which does fade to green instead of to brown. When new the two colours were almost identical (not quite, though). The other option, certainly for an early tank, would be SCC 2, which is a medium shade of chocolate brown. This was superseded in 1944 by SCC 15, but vehicles were only to be repainted when the old paint was in bad enough condition to make this necessary, not just to get them in the new colour.