Just fyi, as far as I know, there is no test to test against all horses to see who their breeding is. You have to have a "Potential sire" and "potential dam" to test against. There is no centralized database for horse genetics.
Well, IMO he looks like a dark buckskin, almost borderline sooty. We had a buckskin colt that was nearly as golden brown as your mustang was and he also had a little bit of white "frosting" in his hair and we did the creme gene test on him and he tested positive for it, so I'm pretty sure that your mustang was a very dark buckskin if he had white hairs mixed in with his mane.
Yup I see a gold tint, though he is dark like a bay he still has gold shining through, I say possibly a very minimal Sooty/smutty Buckskin ;) though others may disagree and you'll truely never know for sure. Though it is interesting to see the different colors, and YES mustangs do come funky colored lol my girl sure is and I am VERY tempted to get her tested just to know lol