There are several varieties of sex-links. Sex-links are not a breed. They are a hybrid created by breeding a roo of one breed with a hen of another, the result of which are chicks that can be sexed by their color or markings. The sex-links with which I am familiar have been bred for two purposes: 1. to combine and enhance the excellent egg-laying abilities of the the parent stock, and 2. to tell the boys from the girls immediately upon hatching by their color or markings, thereby eliminating the inaccuracies inherent in vent or feather sexing.
Because they are not a breed, sex-links will not reproduce themselves and you won't get the benefit of sex identification based on color. To get true sex-links, you have to start with the parent stock that produces the type of sex-link you want. If you simply breed two sex-links, you could end up with many combinations of mutts. Not necessarily bad chickens, and quite possibly some very good chickens, but the unpredictability makes it an undesirable way (to most people) to obtain more chickens.
UGCM