Parah, Chapter 2, Mishnah 2. This chapter is devoted to the physical features that make a cow fit or unfit to serve as the parah adumah. Our mishnah lists several conditions that at first glance look like they should disqualify the cow, and explains which of them truly matter.
Black horns and hooves
"Parah shekarneha utlafeha shechorim, yagod": if the cow's horns and hooves are black, the black portions are simply cut away, and the cow remains fit. The blackness is trimmed off and the cow's redness is not compromised.
Places where blackness does not matter at all
"Galgal ha'ayin vehashinayim vehalashon einam poslin baparah": the eyeball, the teeth and the tongue do not disqualify the parah. Even if these are black, the cow is fit, and nothing needs to be done about them.
A dwarf cow
"Vehanenneses ksheirah": a dwarf cow, one that is abnormally small, is fit. It is worth noting the contrast. In a human being such a condition counts as a blemish, and a kohen who is a dwarf is disqualified from serving in the Beis Hamikdash. In an animal, however, this is not reckoned as a blemish, and the cow may be used.
A wart that was removed
"Haysah bah yabeles vachatachah, Rabbi Yehudah posel": if the cow had a wart on it and the wart was cut off, Rabbi Yehudah rules that the cow is unfit.
Rabbi Shimon takes a narrower view. He speaks of any spot from which something was cut off, "kol makom shenital", and he rules the animal out only where that spot afterward brought forth no "se'ar adom", no red hair. So if red hairs came up over the site of the excised wart, the cow stays fit; she is unfit only when the spot failed to raise red hair.
Rabbi Yehudah disqualifies the cow even where red hair did grow back in that spot, because hair that regrows over such a place is not perfectly red, and hair that is imperfectly red is itself considered defective.
The halachah follows Rabbi Yehudah: once a wart has been cut away, the cow is unfit for use as the parah adumah.