M966 and the M1036 (an M966 with a front winch) have both been around for a very long time. They've seen service in both wars (both US Army and Marine) although they are being replaced in active service by up armored variants.
The interior remains the factory forest green color. Some very late HMMWVs that are manufactured in tan have a tan interior. These late HMMWVs would probably be the uparmored types. Most likely UN HMMWVs are taken from current stocks, overpainted white on the outside and sent into theater. The interior would remain the original factory color, be it green or tan.