Registration papers should be proof, but without DNA confirmation one can never be positive. I know a breeder (in another breed) who was getting old and a bit confused and I am sure some of her pups' parents were not accurately attributed, but the pups were purebred, just possibly from different sire. Coat colour can vary tremendously and it's more likely when it isn't a disqualification in the breed. If you are buying a dog described as purebred then there should be registration papers given to you. A DNA test should tell you whether the dog is Basenji or mixed with something else, if you really want to know.