It is on a different locus than the spots gene, and can result in totally white, roan coloured or solid coloured animals. It is represented by R or r.R is roan, r is solid.
RR animals are Homozygous for the roan gene. These animals will be white with the exclusion of some coloured hair inside their or around the ears. These animals can only pass on the roan gene, every single calf they have will either be roan, or white, depending on what genes the animal they are bred with/to pass on.
Rr animals are Heterozygous for the roan gene. These animals will be roan. The amount of roaning on the animal can vary a lot. Some will only have a bit of roaning on the tail, face, hocks or brisket. Some will be mostly white with a bit of colour thrown in hear and there. You can have Blue Roans, Red Roans, Tan Roan, Grey Roan and so on. It all depends on the Base colour of the animal. You can have spotted roans, or white faced roans if the animal also carries one of the spotted genes.
The above 3 animals are all Rr. 1/2 of the calves they produce should be roan when bred to solid coloured animals. Notice the difference in the amount of roaning.
This animal is Ee, ShS and Rr. So she is black base colour (red carrier), Hereford marked and Roan in color.
This guy is ee, ss and Rr. So Red in colour, spotted (recessive gene) and roan.
rr is homozygous for solid colour. These animals will be solid coloured, and cannot pass the roan gene on.