I forgot where I read this, but the theory goes
Dragon x dragon = 100% dragon
Dragon x geno dragon = 50% - 50%
Dragon x non metalic/dragon = 100% geno dragon
Geno Dr x Geno Dr = 25% dragon 50% geno and 25% non dragon
IME, in real life it isn't as simple as in theory. Many factors take part in the development of offspring (such as the numerous genetic background of parents). And this is more confusing if you only get a few fry - which genetic actually survived?
I'm not sure what fancies are, but I'm guessing it involves a number of color genetics combined with a certain mutation. So I don't know what color outcome you will get other than similar to the parents (since they're siblings). If you introduce dragons to them, you will be mixing them with at least metallics (dragons are considered as Full Metallics).
If I may suggest, breed your fancies and get a definite fancy line before mixing dragons into them.... unless you are definitely sure they're genetics are fixed or if you don't mind surprises.
Hope that makes sense.