You can say that all you want. And I understand that giving birth is a strain. But there is no reason for that much rib to be showing.
It's not just that giving birth is a "strain" on their systems... but also that the ribcage widens during pregnancy and will stay wide after birth (for a time... they do usually "shrink" again if she's not re-bred) - sometimes significantly that can cause rib to show regardless of how much weight the horse is really carrying.
Bear in mind that ribs are NOT the ONLY indicator of healthy weight... and according to our vet (and several equine nutritionists) they are the LAST part we should be looking at to determine weight. Like people, horses can carry weight in different places too... (I've rehabbed more than a few horses - working closely with vets and nutritionists - they agree on this subject)
This mare, in all photos has ample fat coverage along her crest, spine, and base of tail. She doesn't appear gaunt... all of which means she's nowhere near being "too thin".
Aside from that, it IS healthier for a horse to be on the leaner side rather than the heavier side - just like it's healthier for people. Horses are natural athletes!
In regards to the foal color - I'd guess dun... a DNA test would give you the answer though :) Lovely mare, and cute foal!