I'm not an HVAC Guy, so most of my experience with them is fixing the house after the AC system fails, or leaks from not having any bleach poured through the drain lines. There is a big Trane factory here in town and from what I've been told by people that work there, I wouldn't own one.
Sigarms is the HVAC expert on here. Hopefully he will see this and comment soon.
I've never seen any evidence that the color of the shingle affect the temperatures inside the attic. One thing that people sometimes get confused is that the heat in the attic is needed to create air flow through the attic. Air rises when it's hot, and that hot air in the attic pulls cooler air from the soffit vents, into the attic, and then out through the roof vents. In my opinion, a ridge vent gives you the most air flow. More air flowing through your attic, they dryer it will be. Air flow is how condensation is removed from an attic. A dry attic will last forever.
Where I live, R40 is the minimum R Value for Code. I personally have R60 in my home. Simple rule of thumb with blown in insulation is R30 is about a foot thick, R60 is two feet thick. After R60, you are not gaining anything more. Here, our main battle is against the heat, and the higher R value that you have in your attic, the less you pay to cool your house in the summer.
For your area that is limited to the thickness of the 2x12's, which are 11 1/4 inch tall, the very best you could hope for is R30 if you use fiberglass batts. That's OK, but it's not great either. If you can, that would be an ideal place to go with closed cell foam. NEVER use open cell foam on a roof, it retains moisture and speeds up the decay of the roof.
