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 cock 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.