Four Lane, one of the last Bookcliff stallions, was a very nice grulla roan. I hope that the above information and bit of geneology will help you when you are looking for an authentic Spanish Mustang stallion for your mares. There are many to choose from, and all of them of the highest quality available to the public. So unless a herd was in total isolation they did not escape this fate. Then add to that the many horses that were released or escaped into wild herds and you will realize how difficult it is to find horses that meet with the strict criteria set up by the SMR founders.
Many wild horses retain the look of their Spanish ancestors.
If I breed my SMR stallion to a Quarter Horse, Appy or Morgan, the resulting foal will often look so much like a Spanish Mustang that it could easily be mistaken for one, Spanish Mustang genes are very strong and many of these breeds already carry Spanish blood. As far as the DNA question is concerned, the problem is, it is impossible to know where a horse got it's Spanish markers from. It could be a registered Quarter Horse and still show Spanish markers. Even English Thoroughbreds carry Spanish markers. To be a registerable Spanish Mustang, your horse must be of genetically viable Spanish Mustang blood.