I was trying to think of things that are artificially dyed red and salmon came to mind.
Farmed salmon would be gray without the dye they are fed because they don't eat their natural ocean diet of krill.
I know some other meats are dyed red to make them look more appetizing to people.
What are these dyes made of?
Via the blog
For the most part, farmed salmon are simply fed synthetic versions of two pigments of the carotenoid family -- astaxanthin and canthaxanthin.
Wild salmon take in the naturally occuring versions of these carotenoids by virtue of their aquatic diet.
Farmed salmon -- subsisting mainly on grains and corn -- need these dyes added to their feed so they can have a pleasing rosy color.
This is mostly done for aesthetic purposes.
Would you be interested in taking home a filet of salmon that was completely gray? No, you wouldn't. And salmon farming companies know this very well.