Yes my mare changes seasonally. She's a lot darker and more solid in the winter, though you can still see white in her, just not as much. When she sheds out in the spring is when it is most obvious. So, your saying that my filly could shed out sooty at any time in her life? It's not an every year kinda thing? The only horse I know that is sooty is her sire, and he seemed to get darker each year. But I do remember him being light when he was young like she is. Only difference, whenever he got bit or scraped, his foal coat came in solid black in those spots, sort of like a roan.