The name Sex links comes from the sexlinked traits that are passed from the parents to the chicks.
For Red Sex links ( aka Golden Comets, Gold Sexlink, Red Stars, etc...)
You must breed a roo with gold genes ( RIR, HHR, BUff, Partridge, etc...) to hens that has silver genes ( RIWhites, Delawares, SLWyans, etc. are the best ones to use, most White leghorns and some White Rock don't have the silver genes so they wouldn't work. This cross results in chicks that can be sexed by color at hatching, males are white, females are red. Here is a thread with beautiful Golden Comet hens and a GC roo.
https://www.backyardchickens.com/forum/viewtopic.php?id=272809
With the Black Sexlinks, You can use any colored rooster as long as he is
not Barred and
not dominant white (ex.White Lehorn)or masking barring under the white( White Rocks). But any Red, Buff, Black, Blue, Splash, Partridge, Laced, ect roo will work though hatcheries use Production Reds mostly. This cross with give roos that have a white spot on their head and will be Barred and the hens will be solid black was chicks and will be a solid color with maybe red coloring in the breasts as adults.
Here are some sexlinks that I bred from a Buff Orp. roo on Barred Rock hens.
Dayold pullets
Dayold roos
5 wk old pullet
5wk old roo
5 month old pullets
5 month old roos