Appaloosa varnish is a progressive silvering effect that leaves colour (called varnish marks) on the bony prominences of the face, lower neck, forearm, knees, hips and stifle. It is caused by the SvS allele. This is probably semi-dominant: it’s thought that horses homozygous for the allele become lighter than heterozygotes.
The pattern is sometimes called marble, marble roan or varnish roan. However the pattern is not roan and, much like with gray horses varnish horses are born another color (often with another appaloosa pattern), with just some varnish on the head, and the silvering develops with age. Unlike with gray other appaloosa spots usually retain pigment. The silvering can occur over the entire body or just over the rumps, when it may be mistaken for frost, though it is actually a varnish blanket.