Yeah, Ford -- indeed, pretty much every manufacturer -- has some goofy rules about interior/exterior color pairings. Granted, there are some combinations that would raise eyebrows, and maybe Ford prohibits those because they think it would reflect badly on Ford, but as has been mentioned there are combos that would be harmonious if only they were allowed. But for some reason, they aren't.
I don't understand the reasoning; and it makes even less sense in a car like the Mustang in which the only thing they actually change are the seats and door inserts (and it looks like they don't even change the latter on the 2015s).
EDIT to add: I was just playing with the Build & Price tool, and if you try to pair a Saddle interior to a prohibited exterior color, it does try to force the exterior color to Black. However, if you go back and change the exterior color first, there are other combos that are allowed ... you can get a Saddle interior with Deep Impact Blue, Guard, Magnetic, Oxford White, and a couple others. The Build & Price tool is a little confusing in this regard.
2012 GT Premium Coupe | Performance White | 401A Charcoal+Grabber Blue | Comfort Pkg | MT82 | Boss 302 Shift Knob | 18" Pony Pkg Wheels w/Brembo Pkg Center Caps | Boss/CS Front Valance | Scott Drake 5.0/Tri-bar Fender Emblems | FRPP Strut Tower Brace | Redline Hood Struts | GT500 Rear Splash Guards | Rear Spoiler Delete | Sound Tube Delete