Rare yellow face? What do you mean? Can you show pictures? Do you know their parents mutation, because they are most likely split for something. Also, is the skyblue the **** or the hen? I think we need a bit more info
I agree we do need more information but if you breed a normal skyblue to a normal blue yellow face you will get...
and about half yellow face half white face
But you may also get more mutations depending on what the pair is split for.