Mitchells in RAF service were finished in Olive Drab and Neutral Gray. The OD faded quite quickly though and it's probably best to mix your own to suit.
Areas where US markings may have been originally supplied, such as 'stars and bars' and tail numbers, were painted out in RAF Dark green
I have some B&W pics i purchased from IWM some years ago, but I seem to have misplaced them !
However, here are a couple from various publications, again from IWM, now in the public domain, and more can be found by searching for 'RAF Mitchell'
I also have a thread from a couple of years ago, in one of the GBs, where I used the old 1/48th scale Revell kit to make a Mitchell II of 98 Sqn, with the waist windows - something a little different from the 'standard' Mitchell II and III - and an 'in progress' pic, plus a pic of the actual aircraft, are shown below. Note where previos markings are painted over.
I do have a number of other pics of RAF Mitchells, but they're difficult to scan without damaging the spines of the books concerned.