I need to replace a roof due to hail damage.
The roof currently has a grey shingle.
We have decided that a charcoal gray (almost black) shingle would make our house more
attractive.
However, we are concerned that the darker color will make the attic and upstairs warmer. Question: Is there a "rule of thumb" to apply, such as a black shingle will raise an attic temperature by "x" degrees over a lighter colored shingle.
Thanks!

It's really a non factor. Your attic should have ventalation.