the inheritance mostly works like a dominant gene, so understanding what causes yf is not necessary to work out the breeding results luckily... as nev said there is some disagreement.
the only difference from a normal dominant-recessive inheritance pattern i can i think of is with yf1 where a df yf1 has a white face like a non-yf1. the other two act just like you would expect a dominant gene to.
i think there are two theories as to what a yf actually is. some think it is a mutation of the blue gene so that it does not completely lose the yellow. if this is true then a green bird cannot be yellow face, but it can be green/yf blue.
others think it is a completely separate gene that causes the yellow to appear on the blue. in this case any bird, including greens, can be yf but obviously it is not visible on a green...
these differences do make a bit of difference to inheritance but not enough to really confuse things.