Houzz is the new way to design your home.
For comparison purposes, here is good example of a cape style house with dormers and a variegated "black" roof. I believe this is close to the onyx that Jim has selected. Very classic and high contrast, stately.
This roof takes the contrast down a notch, which I am drawn to. It is a dark variegated gray.
It seems the lighter the color, the "warmer" the house begins to feel. Less stately, and more cottage-like.
These shingles are literally, warmer in color, with red tones. It emulates a natural shake shingle roof. A very different look, again more cottage feel.
The next two images show some metal roofs on cape houses. In general, I like metal and have even seen the successful use of some metal and some shingles on a house. However, this is usually a more contemporary application. In this case, I am inclined to suggest sticking to one material, as it will help the porch read as if it has always been there.