I think Ss would be correct, because it is saying that it is heterozygous for the trait. For example PP would be a super Pastel, so Pp would be a normal (heterozygous) pastel.
And I just noticed the title, this is not a double co-dom this is codom/dom because there is to "Super" (homozygous) form of a spider.
Pastel is co-dominant
Spider is dominant

As always, Correct ME if I'm wrong!!