One of the easiest Fire finches to be sexed visually. It is one of 4 subspecies of the Black-faced Firefinch (regarded as a separate species by some) In this sub-species the bird has a  pink breast (cock left)

The cock has a jet-black face, whereas the hen has a brown face, reminiscent of that of a Peter Twinspot hen (cock left)                                               

In this sub-species the crown is also lighter than in the nominate form (cock left)                                               

Lastly the cocks show more colour on the back and wings (cock left)