Grey is a pattern of individual white hairs intermingled with colored hairs. It can occur on any base color. Grey foals are born colored and become progressively lighter with age until the horse appears white, but the grey horse retains its dark skin, unlike a true white horse, which has pink skin.
Grey is caused by a dominant gene, (G). GG and Gg horses are grey; gg horses are not.
Roan is similar to grey in that it is a pattern of individual white hairs sprinkled into the coat. Unlike grey, roan horses are born roan and stay the same color throughout their lives (although they may appear lighter or darker in their winter coats). Also, the head, legs, mane and tail of a roan horse are solid-colored and do not display "roaning." This makes roan easy to distinguish from grey, since the face of a grey horse is the first part of the body that begins to lighten.
Roan can occur on any base color and is caused by the dominant gene R. Homozygous roan (RR); Almost all roan horses are heterozygous Rr, while rr horses are nonroan.
Since roan can occur combined with any color, the appearance of roan horses varies greatly. The common shades of roan are Blue Roan (roan on black), Red Roan (roan on bay), and Strawberry Roan (roan on chestnut).
Click on the above picture
Cedar Ridge Quarter Horses
has a very informative site on the genetics and inheritance of Roan
