Hi Guys,
Sorry to say, I'm not sure what color was used in the P-400 cockpit. The records are slim on those pre-Lend-Lease orders, though quite a few of them show British Interior Grey-Green on other aircraft. I've found a number of pre-war documents discussing commonality issues between the P-400 and the P-39D, but none of the docs discuss colors.
Contemporary photos show P-39s with both versions of Dull Dark Green in their cockpits - that is the black-green version and the deep forest green version. I've not seen any Bronze Green evidence, but there's still a lot I've not seen. (There certainly was leftover Bronze Green used as a finish coat on some landing gear - and leftover paint might have previously been used in cockpits.)
Very late P-39 production used Interior Green cockpits, but those aircraft were built long after P-400 production had ended.
Last note on "Bell Green" - so far all evidence of its existence is anecdotal. I've never found a written explanation of it's existence, and the anecdotal evidence points to three different colors.
Wish I had more on this - I'd love to know more on export colors for my own projects!
Cheers,
Dana