By the way, I hope you mean 'about' a member of the royal family, not TO a member of the royal family. People can be locked in the Tower of London and have their heads chopped off for insulting royals, you know.

Do you mean 'purebred' as opposed to mixed race/hybrid/mongrel? Like when we are talking about animals?

It would be understood. "Purebred" refers to animals. It is of course irreverent and disrespectful to compare a royal lineage to an animal pedigree. Still, one could certainly say something like "you could see from his lip that he was a purebred Hapsburg" and be clearly understood. If a native speaker said this, it would be understood to be an intentional joke. If someone with an accent said it, people would be likely to think that he had simply made a mistake and used the wrong word. In "A Tale of Two Cities," Charles Dickens wrote "There were a king with a large jaw and a queen with a plain face, on the throne of England; there were a king with a large jaw and a queen with a fair face, on the throne of France," which is somewhat disrespectful--but note that he was writing about DEAD kings and queens! By the way, it appears that the last execution conducted at the Tower of London was on 15 August 1941, but he was executed for spying, not for insulting the King--and by firing squad, not by having his head chopped off.