This is very much a personal preference, but you can cull for the obvious things- cull meaning remove from the breeding pen, not necessarily kill. Some of the easiest things to look for on chicks are non feathered legs, or feathering on the middle toe. These can go to a layer flock for dark eggs if they are hens, or if roosters you can find someone who wants them for an olive egger breeding pen. As they grow you can pull out those with defomities like side sprigs on the comb, wry tail, or other serious DQs. Leg color I seem to be able to be sure of by about 6 or 8 weeks. A really bad tail set you should be able to pull out at about 3 months (or sometimes keep one of these as backup in case the coloring in your other birds is bad). Otherwise yes you have to wait for the coloring to come in at 6-9 months to truly pick your breeders.
But I do not have a lot of personal experience with FBCM; I'm interested to hear how other breeders run their culling program.
But I do not have a lot of personal experience with FBCM; I'm interested to hear how other breeders run their culling program.