Shrimp shells turned gold not pink in stock?

Got some frozen shrimp, not the shoprite brand I usually get, but still basic ez peel frozen jumbos, nothing fancy. I used the shells for a quick stock (pasta water to use in my sauce)

The shells turned golden? Never seen this before and was just curious, they're usually a solid pink after cooking. Everything smelled fine after defrosting so I'm not really concerned about safety. Just curious why?