What pigment makes salmon red?

Salmon's muscle tissue is vascularized with blood vessels and is built for long endurance swims as opposed to white muscle which is built for short, fast bursts of speed.

If you are talking about their skin pigment then it is colored by a carotenoid-class antioxidant pigment called astaxanthin … as do lobsters, crab, shrimp, and other shellfish with reddish parts. Salmon and shellfish get astaxanthin from eating plankton (or creatures that food on plankton), which get astaxanthin from feeding on micro-algae that produce the carotenoid in the first place