Select a slightly lighter color for the shingles so the shadow line on the shingles is more apparent. It will provide more definition to the roof surface

Too bad you can't do the total roof standing seam. They last a lifetime.

We are doing the same - GAF Timberline HD in Charcoal with black standing seam metal on the porches and roof/cornice returns. I recommend you open up one of the shingle bundles and pull a few of them out to get a feel for how it really looks. Our actual shingles have much more gray in them which is what we wanted. The demo piece on display in the store was completely black and not indicative of the actual product at all.

Seeing the actual shingles will help you better pick that metal.

How did your house turn out? We are doing brown wood columns with a blackish grey shingle and a black metal roof over the porch. How did your turn out? Did you go with the charcoal grey?