I love every one's buckskin horses! I love buckskin's! I used to own an american saddlebred buckskin mare who was just a doll, she had the same frosted looking main as your horse. I called it her highlights, I asked my trained and I was told it's common in buckskin horses to have highlights looking gray in there main because of there body color.
Kinda weird about your horse not liking mud either, my mare is an absolute princess and only rolls when she's shedding her winter coat! She never gets dirty if she can help it. Gotta love a clean horse. My paint gelding who has lots of white of course, loves nothing better than wallowing in the mud.
I think there can be more answers, and one of them is: Fjord pony cross. Fjords have this "two-coloured" mane and when you cross them with other breeds, they sometimes give this mark to their foals. I know several fjord crosses and all of them have this.
Bella - Yeah I don't know why he dislikes walking through mud so much. He's got this "Ick ick ick ick" look on his face the whole time. But he rolls ALL the time. Seriously probably at least 2x a day, maybe more.
I've heard a couple different things about the "frosting" in the mane, one that it is caused by the cream gene (which makes a bay a buckskin) and the other is that it is caused by the dun gene (which makes a bay a dun). I have no idea what it is though... I've seen several buckskins and duns (Soda) with the trait and several without....
I think it's probably genetics or something like that. But is he spooky or stressed a lot? One of our horses is a little more high strung and gets spooky and stressed more than the other horses we have (don't get me wrong, she is a super sweet and lovable horse. We've won a lot of things with her) and she has more gray hair than the other horses even though some of them are older than her. We think that because of some of her stress levels, she has more gray hair. But who knows really. Is could be like humans, some get gray hair sooner than others.
That is a pretty common thing with buckskins and duns. They are called guard hairs and I don't know if there is some particular gene that causes them or if it just has to do with the shade of buckskin the horse is. Also, it can't be due to whether the horse has paint breeding or not cause I know tons of pure QH buckskins with extreme frosting. Just like my horses. Both are registered QH.