As far as I know, there are a number of species of green lings. The greenling shown is a kelp green ling. They, and all other green lings, are cousins of the ling cod. The ling cod is usually brown with white flesh, but it often has a blue or greenish hue in both the skin and flesh. Most of the blue ones I've caught have been smaller, but I know that there are big blue ones too. I don't know what causes the variation, but they both occur in the same areas, so probably not diet. The blue flesh turns white when cooked and tastes exactly the same as the other ones.