That's obviously countershading, it's 'smudged' all over her topline IMO.
I think the OP's foal's dorsal stripe is reliable because it's just as prominent in his baby pic, where he still has his textured baby fuzz, as it is in the picture when he's in the trailer and much older and is done shedding his foal coat.
Just as texasgal said, he's shedded his baby coat, and there's no color change.
@bird3220; chestnut and sorrel are the same thing. The difference between the two varies by where you're from and what terms you grew up with, but it's the same thing, but the correct name is chestnut.