Codes for 2008 mustang
Color Description Vehicle Part
D3 Colorado Red Exterior
G5 Alloy Met Exterior
G9 Vista Blue Met Exterior
HP Hi Performance White Exterior
JV Sangria Red Met Exterior
P3 Windveil Blue Met Exterior
PX Highland Green Met Exterior
U3 Valencia Exterior
UA Ebony Exterior
YN Silver Met Exterior
ZY Vapor Silver Met Exterior
No doubt the '05+ GTR is a custom mixed color (non Ford), but I'm sure you could come close at a paint supply shop From pictures I've seen, it appears a bit darker than Grabber Orange.
From the net:
"The bright Valencia Orange car – inspired by the famous Grabber Orange 1970 Mustang Boss 302 Trans-Am race cars (the Yellow Mustangs)"
Valencia/orange. Code is U3.
There are 2 shades. GTR look slike the darker shade.
You could just have a paint shop mix up some U3 and darken it a lil bit.