Here is what I've recently learned. The alleles (?) for green, blue, YF and GF all sit at the same spot on the gene. The order of dominance is: green>GFblue>YF2blue>yf1blue>blue
So, if you bred a light green bird (not split to blue) to a DF YF1 sky blue, for example, 100% of the babies would be light green split to YF blue. So if you then bred one of those babies to a blue/blue bird, all the blue babies would be YF blue because they'd inherit type 1 YF blue from one parent and plain old blue from the other. Make sense?