I dug up this related thread last night regarding Olive Green Tigers in Tunisia. Lots of good informed posts, but still no definitive hard answers.
http://cs.finescale.com/fsm/modeling_subjects/f/3/t/86794.aspx
BTW, those Bison decals look awesome- too bad they are OOP and sold out, they would be perfect for my build. My prime reference source, “The Tiger Tanks” lists the 1st company of 501st being sent to Africa originally in overall Desert Brown with white outline turret numbers and later in Tunisia they had the red centers added.