What color horse will you get if you breed a blue roan to grey?

The Jockey Club definition of roan is essentially the same as grey...which would mean that there would be a 75% to 100% chance of a horse that gradually turns white over a period of years reguardless of it's base coat color.

If roan has true roan (Rn) genetics and the other parent is grey (G) we know that each parent has a 50% chance of transmitting their dominant gene (if both are heterozygous). This means that 25% chance of Roan 25% chance of Grey 25% chance of both Grey and Roan and a 25% chance of the foal being the base color only (base color could be black, chestnut, bay etc based on the parent's genetics at the Agouti and Extension sites).

If one parent is homozygous the foal have that characteristic 100% of the time and the foal will also have the other characteristic 50% of the time.