There are points during a pregnancy that it is impossible to accurately palpate, if the foal is carried low and forward, the vet would never feel it. Ultrasound is your best bet when you don't know how far along they could be.
As far as color, your mare is a fleabitten grey. When such colored horses are in winter coats, their fleabites hide and the horse appears whiter. When they shed their winter fuzzies, their fleabitten color appears. Same thing happens with my sister's fleabitten grey mare ;)
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