I think you might be lucky, that the colt did not inherit the greying gene, however, there are cases where a horse with the greying gene, showed little, if any, until he was a few years old
I would say wait until he sheds out as a yearling, to be more sure.
Changes in the color of gray horses[edit]
A gray mare with suckling foal. The light hairs around this foal's muzzle and eyes indicate that it will gray like its mother. Not all foals show signs of graying this young.
A gray foal may be born any color. However, bay, chestnut, or black base colors are most often seen. As the horse matures, white hairs begin to replace the base or birth color. Usually white hairs are first seen by the muzzle, eyes and flanks, occasionally at birth, and usually by the age of one year. Over time, white hairs replace the birth color and the horse changes slowly to either a rose gray, salt and pepper (or iron gray), or dapple gray. As the horse ages, the coat continues to lighten to a pure white or fleabitten gray hair coat. Thus, the many variations of gray coloring in horses are intermediate steps that a young horse takes while graying out from a birth color to a hair coat that is completely "white."