Short answer: Yes, it's legit.
Long answer: Like the Mustangs and P-40s delivered to the RAF, these Martlets (or G-36A to the French) were painted in US equivalent camo colours before delivery overseas. The scheme is meant to emulate the Tropical Sea scheme, not the Temperate one we are used to seeing on most FAA/RN aircraft. The British colours used would have been Light Mediterranean Blue and Dark Sea Green over Sky. Sky was relatively new in 1940 and not readily available yet. Grumman would have had equivalent paints for Sky Grey and Sky Blue, both pre-war RAF colours. Since Sky was colloquially described by the Air Ministry as a 'duck egg blue', Grumman probably chose Sky Blue for the undersides. It is a weird scheme, perhaps they were originally planned on being deployed in the far east. The earliest of the French Martlets had the full rudder stripes as seen on some allied aircraft during the BoF. My references have a profile of one of these, before the introduction of the Night marking, so it could be done.
I skinned this one for IL-2 a couple of years ago using the F4F-3 model, since the Martlet MkI had a different gun arrangement I consider it a 'hack'. Never got around to finishing it. BTW, the Sky fuselage band shouldn't be there, at the time I was using this skin to get the band placement right for another skin I was working on.
