I notice that the rear compartment on some M998 Cargo/Troop Carriers are camouflaged to match the exterior 3-colour NATO scheme, while others have the area painted in a single NATO Green colour.
Are both options acceptable, or could this be re-painting done on privately owned vehicles?

Both ways are seen on US Army vehicles. Usually, if it is an open rear (pick-up truck style) the bed will be painted in NATO camo as the outside is. If it started life as a 4-seater with the bed area inside the hard or soft top, the bed area is painted green along with the rest of the interior. If the enclosed interior version is changed to an open rear version, the bed is not repainted camo unless the whole vehicle goes through a repaint.