There is a specific white head marking with a partially white bottom lip. Because the horse's left side is affected, it only really becomes visible on adults. However, on the live server, a tiny bit of pink skin is already visible on the foal's bottom lip, too.
This seems to have disappeared now. On the beta, the foal has no sign of pink on its bottom lip. Looking closely, I think it would make sense for a tiiiny speck to be visible?
Attachment 1: Foal from the live, if you zoom in closely, you can see the tiniest bit of pink on its bottom lip.
Attachment 2: Foal from the beta server, same head marking, but no pink on the bottom lip.
Attachment 3: Same foal grown up, now showing the white bottom lip marking.