Hmm.... how about flat black with a reflective black edge
Most of my stickers are either white or silver (to match the stock trim).
I've currently got a 'Life's Good' decal on my front fender that's sort of
khaki (it's starting to fade and will probably be removed soon). This might give you an idea of what khaki looks like against the Nat green color, though:
here's the decal: