Xavier, you can generally tell if a pigeon is a purebred by looking at how well it conforms to the breed standard, but not always.
A lot of breeders have crossed breeds together to get desirable traits more quickly in the breed they are working on.
Some examples are:
Crossbreeding other breeds, of rarer colored pigeons, into flying rollers, to produce rare colored flying rollers.
Crossing racing homers with kings to make more prolific kings.
Crossing long face, clean legged tumblers to saddle muffed tumblers to create saddles with better frontals and shorter beaks.
Crossing modena and long faced, clean legged tumblers into show rollers to produce larger and more rounded body show rollers with rounder heads.
Standards costantly change as people interpret them differently too.
Breeders take the crossbreeds created and breed them back to birds from the parentage they are working on until they meet the standards.
Keith