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Salmon meat is reddish-pink due to eating krill and salmon, which contain astaxanthin, and dies their meat.
Farm-raised salmon are never exposted to astaxanthin, and have naturally grey meat, but most grocers add dye to make them look more "natural."
Bonus fact: astaxanthin is why flamingos are pink (they're born with grey feathers, but eat lots of shrimp)