Which rooftop bar is the best to see sunset through the porticos of the colosseum?
To be honest, I've not heard of a sunset view of the Colosseum being a thing. If you don't get any suggestions, would you be open to some different ideas? Time of year could make a difference too; not all rooftop terraces are open in every season.
Just for kicks, google up some sunset pix from Giardino Degli Aranci (Garden of Oranges) on the Aventine? It's not a bar but bring your own wine, a snack, and enjoy the show (assuming it's not a cloudy day.)
So, to see the sunset through the Colosseim arches, you need a place more or less due east of it. The closest thing is a block away and is a patio bar, not roof top. But it looks straight at the colosseum, over a set of sunken ruins. It's called The Court.