There is technically no such thing as a purple roan horse. Roan is a term used to describe a horse's color when the horse has one color of body hair but has white hairs interspersed within the body coloration. The most common type of roan is a red roan or "strawberry" roan. This usage of "roan" referrs to horse when white-hairs are interspersed on a chestnut body coat. Blue roan refers to the color when a horses has white hairs interspersed on a blacken body coat. Even though there is such a thing as a red roan and a blue roan, there is no such thing as a purple roan.
Because roaning patterns can give a tint or hue to the horse's coat depending on the shade of the horse's base color, you may have heard the phrase "purple roan" or "lilac roan" in reference to a horse that is actually a blue roan.