I didn't really have a choice in the matter, I picked up a local second-hand one and I was more concerned about making sure it wasn't stolen than aesthetics. It's the old gloss racing green but now has a black stem and seat post (non-Brompton). Racing green in gloss has a much more old fashioned, 'classic' feel about it and it can either look heavy-tractor-ugly or quite stylish depending largely on whether it has a brown Brooks saddle to complement the (deep, warm) green. A honey saddle clashes and a black saddle doesn't enhance the green.
I prefer the newer matt finish, generally, though I'm not sure it makes sense with racing green. Looks too military, especially with black accessories.