I would say there's no way California gets cold enough for you to need to worry about the coop being too cold. If you're worried about it getting too hot, you could always put in some sort of barrier between the tin and the roof. Even a tarp would probably work, or if there's room in your budget, plywood. I don't know if they sell Onduline panels in the states, but I decided to try those with my rabbit hutch. They look great, and they were super easy to install. I don't know how they behave in a hotter climate like yours though, some say they may drip some bitumen in the beginning, and you wouldn't want that on your chickens, so you would probably need something underneath them. But as mentioned by others, a light colored tin roof would probably work just fine for you, as long as you make sure your coop is well ventilated. In your warm climate, you could even put up a whole wall made of hardware cloth, facing away from the wind.
*Edit* Well, I should probably have noticed the second page before making suggestions. Looks great.