So the father could be either a Light Brahma or a Buff Brahma? Here are the possibilities I see, given the hen breeds you already mentioned.
If the father is a Light Brahma, the mother is probably a White Leghorn but possibly a Buff Orpington.
If the father is a Buff Brahma, the mother must be a White Leghorn, not a Buff Orpington (2 buff parents would produce chicks with buff feathers, with or without black markings.)
So if you don't know which is the father, the mother is probably but not certainly a White Leghorn.
The mother cannot be a Brahma, because any combination of the available Brahma colors would give a chick with black markings.
If the father is a Light Brahma, the mother is probably a White Leghorn but possibly a Buff Orpington.
If the father is a Buff Brahma, the mother must be a White Leghorn, not a Buff Orpington (2 buff parents would produce chicks with buff feathers, with or without black markings.)
So if you don't know which is the father, the mother is probably but not certainly a White Leghorn.
The mother cannot be a Brahma, because any combination of the available Brahma colors would give a chick with black markings.