Blitzwing - the Cromwell was likely painted in the British version of olive, called SSC 15. It is described as being very close to the American olive paint, the difference becoming more apparent as it aged, and begins to show more of a green hue.
Mike Starmer researches British colours, and here is his formula for Tamiya paints:
5 parts XF81
1 part XF58
1 part XF71
This is what I ended up with using the above mix on a Canadian Firefly:
------------------------------------
Gamera - looks like you are having fun with your project, looking forward to when it all comes together.
regards,
Jack