If it were me and I wanted a clean and minimalist look, I'd choose a bolt/screw with a square drive head, and paint it to match (before installing it). I have seen smooth caps to cover the heads for Ikea furniture fasteners, but they were made out of plastic and relied on forcing a pin on the underside of the plastic cap into the hole in the fastener head. They didn't always sit right, though. I think something like this is your best bet:

Try Googling "square drive screw," or "square drive wafer head screw."

In addition, if you Google "Decorative metal screws and caps," a ton of stuff comes up: Decorative caps