I suspect a white roof made (at least partially) of recycled materials--and 100% recyclable or compostable materials--is your most sustainable option. It will greatly reduce the amount of heat your house absorbs in the summer, which will reduce the amount of energy you need to cool your home. During the useful life of a modern roof, the energy cost of heat absorbed from sunlight is probably a significant portion of the roof's environmental impact. This is especially important if you live in a hot, sunny climate, but even at 50 degrees N white roofs help more than they hurt. Metal roofs are less effective from this perspective than white roofs, but better than dark roofs. Of course, you could always add a white coating to a metal roof.
I don't know anything about sustainable white coatings for roofs, but it's something to look into for the comfort and energy efficiency benefits.