This morning, when I was on my way to work, I saw a bunch of live shellfish being delivered on a TukTuk. So I thought to myself why blue, grey and sometimes greenish shellfish like crabs and lobsters turn red when they are cooked. The process is so common, we never wonder about the chemical process behind it. Well, today, I am going to cover it for you. 

To be simply put, shellfish contains a pigment called Astaxanthin that is in the color red. The pigment is hidden under chains of proteins called Crustacyanin making the shellfish appear dark blue, grey, or greenish when they are alive. However, when they are dumped into a pot of boiling water, the intense heat destroys the protecting protein chains, revealing the red Astaxanthin to the shell surface. 

Source: crabbinghub