I'm not a paint expert, but anything you brush on, will most likely end up with a rough/uneven surface.
I'm not sure its practical, but a sprayed paint will give a much smoother experience. (when done correctly).
Maybe someone with more paint skills will chip in.
I would think whatever shop does your wheels, will be able to make the center cap any color you want.
These, for example, have blacked out centers.
19" Tesla Model S Black Factory Wheels Rims OEM SET OF 4 NEW POWDER COATED SET | eBay