The hairs which progressively grow in and replace base-colored hair in both grey and appaloosa (LP) roan are white hairs.
At a young age/early stages, it might be difficult to tell if a horse is greying or if it is roaning due to LP roan-- the first thing to check is if the horse has a verified grey or LP parent, since neither thing can happen "out of nowhere"-- grey is a dominant gene and a parent must display it for an offspring to have inherited it, as is LP (although LP expression can be very very minimal in some horses.)
There are some visual clues-- Baby greys are often born a deeper "adult horse color" of whatever their original base color is, with white hairs making a 'goggle' like appearance around their eyes. (less commonly a foal who is going to roan quickly and early due to LP roan will have goggles-- not common, but I have seen a couple.)
Some LP horses are also born a deeper shade than normal for a foal coat-- but as LP roaning progresses, the hair left over is often the lighter hair-- so a bay foal might have very few black hairs as an LP roaned adult, making him look like he started out as chestnut.
With grey it is often the opposite-- as a chestnut horse greys, the darker hairs tend to be whats left, making the horse look like he started out a darker color than he actually did.
LP roaning tends to work from the hips and back forward and downward, while greying tends to work from the head and barrel backward and upward. Greying takes body color pretty evenly over the areas it is affecting-- possibly with dapples and fleabites during the process-- and usually leaves the lower legs and mane and tail for last. LP roaning typically leaves definite 'varnish marks'-- remaining base-colored areas over the bony prominences-- nasal ridges, point of shoulder and hip, knees and hocks. LP roans may, less commonly, show some dappling or fleabiting as well.
Grey progresses until everything is white (except possibly fleabites in a fleabitten grey), if ther horse lives long enough-- LP roaning eventually stops, usually leaving varnish marks and darker legs.
Greying affects the spots of a horse who is both LP and grey-- the spots will get more and more white hairs and eventually the hair will be all white if the horse lives long enough-- but app spots are usually the 'last to go' on an app grey, meaning the horse may be all white with charcoal looking spots on its hips for some time. The skin under the white hair on a greyed-out app spotted horse will still show its spotted pattern.
LP roan by itself never makes app spots completely fade to white-- it affects the base color, often revealing MORE spots, as if they were 'under' the darker coat.
LP roans who started out a non-diluted baser color usually have noticeably mottled pink/black or dark grey skin around the eyes, lips, under the tail/between the hind legs, and sometimes in the armpit and groin area and other areas of the body.
Grey horses which started out a solid non-diluted color will have black or dark grey skin all over under their white hair, except where normal white face/leg markings originally were. There is a depigmentating condition called vitiglio which affects some aging greys which resembles LP mottling, but it is relatively uncommon.
LP is not a mutation of grey-- they are located on different genes in different areas of the equine genome.
Last edited by Eastowest; 10-16-2011 at 04:22 PM.