(I have a funny-looking chestnut, we call him a "bucknut" jokingly, because he's purebred quarter, got a chestnut coat and black mane and tail)
One thing I learned here is that she'd need either a dorsal stripe or zebra stripes on her legs to be considered a dun.
Honestly she looks like a dark palo to me, but is that black in her tail? (or just the light?)
If she was a flaxen sorrel/chestnut, shed be a super light one.
To me, flaxen chestnuts look like this
With the distinct red coat.
And duns have the dorsal stripe: (you can barely see it on this horse's back)
This guy is technically a sooty palomino
So I'd defintely say palomino. But it's fun to have an out-of-the-box color like that!