Maybe Emma’s roaning is her Westphalian half expressing itself in weird ways. Neither of her parents was roan, though, so I think she’s just a freak of nature. 
I am not a thoroghbred person but my morgans have some roaning on them as well on the flank mostly.I was told because they are sabino’s that it is one of the traits as well. My filly does not have sabino markings but her father is a sabino so she has the genes and she is roaning a little as well. They are not very very noticable inless you are looking for it. By sabino I mean high white on the legs and a little white snip of white on their bottom lip. We also had a morgan who was a chestnut and had a big plash of darker chocolate colored stain on her side and that was due to a certain line that threw in the dark spots on his foals. Sorry probley not helpful but interesting in the least.
Yeah, our Arab has white spots on him, they end up looking like quarter-sized scars on his hips, etc, but he’s never had injuries there and it’s just part of that bloodline. 
Horse color genetics are cool (and bizarre).
That definitely sounds like a sabino to me…and although not accepted by all breeds, sabino shows up all over the place, including in Thoroughbreds and Arabians. Sabino can express as anything from a few white hairs around the sheath or udder…to a completely white horse.
Andrea, is your Arab a chestnut? Do the spots disappear and then come back? Sounds like birdcatcher spots to me, but sabino is another possibility.
He’s chestnut, a very light/sandy color. Pretty sure the spots are always there, though I can’t swear I see them in his winter coat. I’ll check next time I go home. His dad had white flecking/roaning through a lot of his coat when I saw him with the same white spots, but then, he was also like 20-something then too.
The spots could be due to the water hes drinking. I forgot exactly what its called, but theres a chestnut at my moms barn who has white spots over his body, and his owner says its from some mineral in the water he drank at his prevoius barn :roll: