I don't know much about the breed, but the silver (S) and gold (s+) genetics are the same with any breed. From looking at the Dorking club's site I see their "silver gray" appears the same as a "silver" Ameraucana. Their "red" looks like what would normally be called a "black breasted red" (BB Red), which is wild type at the S locus....just my observations/guesses, so please feel free to correct me where I'm wrong. Both are built on Wild type (e+) at the E locus.
The two varieties are basically the same with the only difference, that I see, being sex-linked silver (S) changes red/gold to white/silver.
Since the females only have one gene for sex-linked traits...
Breeding a red male over silver females produces pure red pullets and
breeding a silver male over red females produces pure silver pullets.
The cockerels from the matings will be "split" for silver and gold, carrying one copy of each gene.
I'm working on bantam Vorwerks and Lakenvelders. They are almost identical except one is gold and the other is silver, so I'll be making similar crosses with them.