Why does meat change colour when it's cooked?

Its down to a chemical called myoglobin. Myoglobin is a bit like haemoglobin, the red coloured stuff that ferries iron around in the bloodstream, except myoglobin is locked up in muscle (and meat is muscle). Red meat contains a lot more myoglobin than white meat, as the muscles that tend to be red are the ones most active in an animal. The legs of a standing animal will be redder, and have more myoglobin, as the muscle has been tuned up for long term activity.Muscles that aren't used as often, fast-twitch muscles, tend to have low blood supply, little myoglobin, and therefore little colour (white meat). Chicken breast and wings don't get a huge amount of use (as chickens don't fly), and so they are white muscle.With fish, most of our salmon has a red colour because we tend to buy farmed salmon, and to keep the meat looking a healthy colour the fish are fed astaxanthin. They would get this in the wild environment from yeast and from algae, it's an antioxidant similar to the chemical which makes carrots orange. Shrimps eat the algae, salmon eat the shrimps and the colour passes through the food chain.