YSL's red-soled red Tribute t-strap sandal.
Louboutin's Lady Derby in black with trademark red sole.
YSL's red-soled Tribute pump in red.
Loubotin's Poseidon in purple with trademark red sole.
I don't think anyone should have exclusive rights to using red on the soles of their shoes. Especially because, correct me if I'm wrong, the only examples of YSL using red soles were for red shoes. If I were making shoes, I personally wouldn't choose to use red soles because it's already been done. But if it's only being used by another designer on red shoes for a streamlined look, then I think it's petty to engage in a million-dollar lawsuit over it. But maybe I would feel differently if it were my trademark.