I didn't know about the sooty thing either till we talked about here in regards to Harley.
Last week when Celia was coming out she noticed the spots he gets on his rump that look appy. She asked me who that horse was:) When I told her Harley she was surprised. She seems to think he has some app breeding somewhere in him. I don't know if I mentioned last year that he not only has that large dark spot on his one shoulder, but he also has a smattering of dark spots over his rump. They showed up last year very pronounced in the spring, then faded to where you couldn't see them. He has them again this spring.
I looked at the picture of your palamino again, and his smuttyness is different that Harley's. Have you ever seen smuttyness that is more spots? Harley has been DNA tested according to his papers, could he still have app in him somewhere? Wish I could take a picture to show you, but it looks like app spots.
Becky
I'm thinking that palominos especially get "protein" spots. At least I think that's what they are called. They develop dark spots when they are very healthy. Appaloosa spots would be lighter than his base coat color, not darker unless they were on a white blanket. Then they might be a couple of shades darker than the base coat.
http://www.equinecolor.com/unusual.html
Bend Or Spots Dark Bend Or Spots ©Equine Color Bend Or spots (also called Ben d'Or, Smuts or Grease Spots). are small to large spots of color that range from a couple of shades darker than the base coat to very dark, almost black. They're most commonly seen on Chestnuts and Palominos but also occur on darker colors, they're just harder to see. These are randomly distributed on a horse. These may not be present at birth, some horses may not get them until they are a few years old and some horses may not get them for several years. The genetic control of Bend Or spots isn't known, more than likely they're a dominant trait.