The actual color of salmon flesh varies from almost white to deep pink depending on their levels of astaxanthin which is a type of carotenoid (organic pigment). The Salmon do not produce this pigment but rather get it from their diet of krill and shrimp. The more Krill and Shrimp they eat, the richer pink their flesh is.
This means with no Krill or Shrimp in their diet the salmon would naturally provide an almost white meat. Since this is not what most consumers imagine their salmon to be salmon raised on fish farms are given artificial coloring in their food to turn the meat pink.