Falb Fee
will breed true, if you have only homozygous Falb Fees in the pen. But, if you have heterozygous, it doesn't breed true.
It's incompletely dominant, so liken it to breeding Splash in chickens. If your flock is just Splash birds, which is two copies of blue, then all you're going to get is Splash. But if your breeding flock also has Blue birds, which is one copy of blue, then you're going to be getting blue chicks, black chicks, and splash chicks.
In this comparison, Falb Fee is like Splash - it's two copies of the Fee diluter gene. Heterozygous Fee is like blue, only one copy, but you can see that they only have one copy. And Pharoah is like black, no copies at all.
Now, you could get something weird popping up because of unexpected recessives, like maybe out of nowhere your flock of splash chickens produces a recessive white chick. I get that, that happens. That's like the one little weirdo chick I hatched, which I'm totally fine with.
But other than that, if all your breeders are splash (homozygous Fee), you're going to only get splash birds. If you're hatching blue (heterozygous Fee) and black (Pharoah), then that means you have blue birds in your flock. And you CAN see that they're blue, of course, so you know you're going to be also hatching blue and black and not just splash (unless you're not educated on how it works).
Here, this is a heterozygous male next to a homozygous male. You can see the difference:
It's not a great picture, since I just took it in my garage with my cellphone, but you can see how the male on the right is more brown/red in tone. That's because he only has one copy of Fee. The male on the left is lighter, because he has two copies.
Don't mind the missing head feathers. They were bullying each other, so they've been split up, but the feathers are still regrowing.