The only way I was able to tell mine apart was by the forehead mark the hatchery put on all of the SS; until they feathered out, that is. I think it was about the 3rd week when I noticed a difference in leg color appearing; the Welsummers were yellow and the SS pink. My Welsummers are also much larger than my SS; and they are all the same age.
My SS were always a deeper brown than the Welsummers. Kinda like chocolate vs. caramel. But it took a couple of weeks for me to notice that. Here are pics of them side by side at a couple weeks old...