Roan is a dominant gene that works over every colour. It is easy to distinguish on dark coloured animals by the distinctly solid base coloured head, solid coloured legs and a brand and any scars which grow back in the base colour. The points above the knees of a roan end in distinct spears up the forearm and this is often the only way to distinguish them on a palomino or buckskin roan, roaning can be very difficult to see on these animals as people who saw the buckskin roan stallion Weston Chilo (imp) will confirm. Cremello and perlino roans are impossible to distinguish unless they are DNA tested or they produce roan foals when mated to solid coloured animals. Grey ponies can also be born roan and grey out before the true colour is determined, some prominent examples are Waxwing Herod (imp), Coed Coch Bari (imp) & Weston Chippendale (imp). Roan cannot skip generations, it can sometimes be hard to distinguish and ponies are often incorrectly registered.
For many years it was believed that roan in its homozygous form was lethal and people resisted breeding roan to roan for that reason. This has now been proven to be incorrect, homozygous roans have been confirmed in Welsh ponies & Quarter Horses using DNA typing.
It can be very difficult to distinguish roan from some heavily ticked animals. In general roans have the distinctly darker head when they drop their foal coat with a distinctly roaned out body colour (this can be very hard to see on palomino & buckskin) and the roaning remains constant throughout life apart from lightening & darkening with change of coat. Ticked animals usually become increasingly more ticked the older they get. They are often incorrectly registered as roan.
If you would like to have your pony tested for Roan we suggest this link.
Only UC Davis in the US does the Roan testing http://www.vgl.ucdavis.edu/services/horse.php