"Color added" on a label for salmon and salmon products means that the salmon were given a feed which contains a pigment called astaxanthin. When the feed is digested, the astaxanthin is absorbed into the fish's flesh giving the fillets or steaks a reddish or 'salmon' color. Astaxanthin is pigment occurring in nature that gives flamingos their pink coloration. It also turns cooked shrimp and crabs an orange-red color. In the wild, salmon get astaxanthin through their diets, which consist of plankton and small fish. Farmed salmon get astaxanthin in their feeds as an added dietary supplement, hence the use of the phrase "color added".
Asthaxanthin provides color to both wild and farmed salmon flesh, but it is also an important nutrient for the salmon, and for humans. In salmon, astaxanthin is important for growth and survival. It acts as a powerful anti-oxidant that protects important fats in the flesh from degrading. It also supports the immune system and cell respiration. For humans, astaxanthin is commonly sold as a nutritional supplement in health food stores for its anti-inflammatory benefits.
Natural astaxanthin can be obtained from harvesting Pacific and Antarctic krill, or by extracting it from shrimp shell waste. It can also be made synthetically from petroleum products, which eliminates the need to obtain it from wild sources. However, because people prefer natural sources of astaxanthin, food scientists increasingly use a micro algae, Haematococcus pluvialis, or a Phaffia yeast Xanthophyllomyces dendrorhous to produce natural asthaxanthin for both salmon and human dietary supplements.