Commentary for Avodah Zarah 47:13

If all you need to do is pass a red cup in front of the mating animal and out comes a red heifer, then why is the red heifer so expensive. The answer is that even two non-red hairs on the entire animal disqualify it. While it is evidently easy to produce a mostly red heifer, an exclusively red heifer is difficulty. So now comes the final and obvious difficulty—why do we need non-Jews to produce the red heifer. If all you need is the red cup trick, then we could just make our own red heifer? The answer is that the trick only works with certain breeds of cows, and evidently those breeds are possessed by the non-Jews. We should note that the Talmud painted itself into this corner with a series of difficulties. This happens from time to time—the Talmud raises difficulties, resolves them but then creates new difficulties that lead to some extremely strange types of resolutions.