Read this thread. Particulary posts 17 and 24
https://www.backyardchickens.com/forum/viewtopic.php?id=422606&p=1
stanglover2001's post is what I was hoping you wouldnt get but I guess that incorrect information comes from the false sources on the net. Though no offense to you stanglover2001 .
Here is why its incorrect though and there is a better explaination in that thread. The Golden Commets are white tailed red columbian, ( like a RIR but white were the black would be). This white comes from Dominant white and few breeds carry Dominant white. So for to make a sexlink this color, the hens have to be carrying both Silver ( for making sexlinks) and Dominant white ( for making the white tail coloring)
White Rocks, are a recessive white breed and in recessive white birds, there is no way to know what color they are masking unless you know their genetic history or if you breed them first. Most white rocks especially the first ones created were masking barring since they were sports from breeding Barred Rocks. But now there are some strains that may be masking Columbian that would be used to make sexlinks, but even then, they wouldnt carry the Dom white unless it was breed in also and without the Dom white, the pullets would be black tailed like the ones on Ideals site so they obviously have a Silver Strain of White Rocks but you cant just definitively say cross and White Rock hen with any gold roo and get sexlinks. So its best to stay away from Recessive White breeds to make sexlinks.
The Delawares, also can not produce a white tailed bird with a RIR roo becuase they also do not carry Dom white, if they did, then they wouldnt show any black. So Delawares when bred with Red roos will produce sexlinks with black tailed hens like RIRs too. like it also shows on Ideals site.
So long story short. The hatchery RIW hens are the only "out" strain of birds that can reliably be used to make the Golden Commet looking Sexlinks without having to recreate a hen strain that would do so . But there are some of these Sexlinks that are 3-4 way crosses that hatcheries have developed a particular strain to use as the mother and father strain that may produce that produce this particular color. So being hatchery birds and hatcheries arent going to tell you what they used for their specialized crosses, then there is really no what to give you a definitive answer.