If the mare only had one dose of gray, she didn't pass it on to her foal. Gray is predominant over every other color.

Our gray mare is heterozygous for gray and sorrel. She's thrown a gray, a sorrel, and a light bay so far. This year's baby is so far a very dark bay, she hasn't gotten any gray hairs in her loins and around her eyes yet like the gray filly did, so she may stay that way. Never get the same color twice from that girl!