Rabbi Eliezer says that a pregnant red heifer is valid but the Sages say it is invalid. Rabbi Eliezer says that the red heifer may not be bought from non-Jews but the Sages say that it may. Not only that, the Sages say that all communal and individual sacrifices may be brought from inside or outside of Israel, from new or old produce, except for the omer and the two loaves offered on Shavuos, which must be brought from new produce and from within Israel.
If the horns or hooves of the red heifer are black, they may be cut off. The eye socket, the teeth and the tongue do not invalidate the red cow (by being black). A dwarf cow is valid. If there was a wart on the cow and he cut it off, Rabbi Yehuda says the cow is invalid. Rabbi Shimon says that if red hair doesn’t grow in the place where it was removed, then it’s invalid.