If I were ordering I'd go Tanzanite Blue with Silverstone interior, with either aluminum or piano black trim. I think Sakhir will be even louder than Fox Red in person, and especially in Full form (which I'd definitely get for the added leather coverage) it will be even more so. The 5 is a big car, so that'll be a LOT of orange.
It's really sad that Tanzanite is only available as a standard option no the 6 Series; I think it would look spectacular on the M5. Not as demure as Imperial but not as loud as Monte Carlo, and has a gorgeous bright undertone in direct light:
http://www.6post.com/forums/showthread.php?t=495046