The Grey gene is dominant, in other words it only requires one copy of the Grey gene to produce a grey horse. Subsequently, to be grey a foal MUST have a grey parent. In order to be homozygous the foal must inherit a grey gene from each parent.
Horses go through many shades of grey while maturing.
For this reason, greys are not normally selected for use in pinto breeding programs. A progressive action, the hair of grey horses gradually turns grey, at various stages many will show dappling patterns. The head area generally starts to grey first.