Every photo I've got of RAF Mitchell II's (B-25C/D) seems to indicate the OD over gray scheme. Some of the RAF Mitchell II's were ex-Dutch Air Force birds, mostly aircraft that had been paid for but not delivered to the Dutch NEIAF before their capitulation.
These were aircraft that had been diverted from USAAC contracts as an emergency measure; the contracted Dutch NA-90's wouldn't be ready in time for the NEIAF to use in the Pacific. So 60 USAAC aircraft were allotted to go to the NEIAF and were definately finished in OD over gray. (As a side note, there was hesitation to divert these aircraft as they were equipped with the Norden bombsight which was still classified at the time.) Some of these Mitchells also went to the RAAF, although some were later "appropriated" by the USAAF.
I've yet to come across any photos of RAF Mitchells which indicate conclusively the application of either of the RAF bomber schemes, day or night. Although I've got a really nice color shot of a Russian B-25 in the RAF night bomber scheme.
Anyhoo, if you decide to use one of the RAF schemes, check
this page out. The bomber schemes are at the bottom. They illustrate a Lanc, but the basic scheme would be roughly the same. As to color matching, don't go for an exact match. There's too many variables involved to get an exact match...
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