The red colour of salmon is due to a pigment called astaxanthin .

A salmon is basically a white fish. The red pigment is made by algae and single cell organisms, which are eaten by shrimps; the pigment is then stored in their shell and flesh. When salmon eat shrimps they also accumulate the pigment in their fatty tissue. In view of the fact that salmon do not lose the pigment, they will turn red in time. As the diet of salmon differs, a tremendous variety of colours exist in natural salmon, ranging from light pink to deep red.

Most salmon these days are cultured. This takes place in large nets in quiet waters (fjords, bays) or in tanks on land. Most cultured salmon come from Norway, Scotland, Iceland, Alaska and Chile. These salmon are mainly fed with fishmeal. This does not contain shrimp, which means the salmon will stay white. The consumer, however, does not want white salmon, even though the taste is the same. For this reason astaxanthin is added to salmon feed. In most cases the astaxanthin is made chemically; alternatively it is extracted from shrimp flour. Another possibility is the use of dried red yeast, which provides the same pigment. However, the synthetic mixture is cheaper

Astaxanthin is a carotenoid, meaning it is a compound similar to carotene. Other carotenoids are responsible for the colour of tomatoes, peppers and carrots. Humans also store astaxanthin in fatty tissues, but we do not get enough into our system to turn red. Flamingos owe their red colour to astaxanthin in the same way as salmon do.