I have an almost 6 month old foal. Started out bay, turned a very dark bay and now has flecks or tickings of gray primarily on his flanks, none on his face, with a few white hairs on his tail head and in his tail. He seems to change weekly. His dam is a gray (started out as a sabino chestnut), sire is black (never thrown chestnut), half sister (same dam, different sire) is a sabino chestnut.
Here is a link to some photos - his half-sister, him with his dam, and some of his face and flanks.
I would love to know if you think he is a dark bay rabicano or is turning grey.