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Old idea, actually. It goes way back to the late 1800's/early 1900's.
Certain breeds, when mated, produce a chick that has unique coloration for male and female, as mentioned.
For example, mating a RIR male to a Barred Rock female gives a male chick that is black with a light spot on its head. The female chick from the same mating is dark, without the spot.
This way you can separate the males from the females at birth.
Great idea for that first mating. But subsequent matings wont breed true to that and you are liable to get almost anything. This is a function of hybrid crossing and holds true for most hybrids.
There are also feather sexed matings, also sex related, ie, females having faster growing, larger wing feathers than males.