when in doubt on humidity with duck hatching, more is always better, food grade coloring shouldnt hurt a thing, but keep it high, 65% threw the whole period is what I alway do. If you run it lower like chickens, you loose a lot of them. I've hatch out 10's of thousands of migratory duck over the years, and you just have to keep it high, misting them with water daily helps too. I keep a spray bottle in the incubators with them, that way the spray is the same temp as the air, dont use cold water! Aside from that, they hatch very well if not shipped, calls that is, or any domestic for that matter.