Black vent covers
An extraordinarily wise saying generally used by interior designers is, "Black goes with everything". Undoubtedly true and something to cogitate about when considering what heat registers will be the most tasteful and impactful. Black vent covers are immediately noncontroversial and as a person updates room decor throughout the years it would be nice to avoid changing out the registers as well, which is definitely a reasonable option if you initially chose a black vent cover.
Black vent covers are available in various finishes, but the best finish in terms of durability for floor use is a powder coat finish, which wins the durability contest hands down. Painted finishes are fine, but are best used on wall or ceiling where scuffing is unlikely to be encountered.
Black vent cover duct preparation
When you look inside a duct opening it is very common to see a lot of unsightly images that would be best overlooked. For example, there is usually shiny galvanized metal or maybe heavily oxidized steel. Perhaps there are exposed stud walls or plaster lath; even uglier would be cured insulating foam with it's ghastly yellowish pallor. On high end installations it is universally common to mask off each duct opening throughout the house and apply a light coat of flat black spray paint. The beneficial effects are immediate and once you place your black vent covers in position you'll find that all the disturbing visuals have mercifully receded into shadow. It's cheap and easy to do as long as you don't attempt this while the furnace is running.
A floor located black vent cover is usually held in place through gravity. However, wall or ceiling applications require use of screws, which are often color matched. Occasionally, horizontally located registers are screwed in place, primarily because of being used in commercial buildings where government ADA requirements are in effect.