IMO the best is to match the walls where they are they are not IMO decor items so best be as invisible as possible

Ok thanks. My door hardware is matte black... and my flooring is antique dirty goat hardwoods with various colors. ... I’m going for a modern industrial look.... so I think I’m going to do black outlet covers.

honestly, I wouln't even do black. just get white or off white. black outlets make me think of aggressively masculine 80s interiors.

You can get paintable outlet covers. Then paint them the same color as the wall so they blend in. But I generally go with white/off white, depending on the colors of the room.

It also depends on the room. In our kitchen, where we have a lot of dark surfaces and the outlet covers are higher up, I have black nickel covers. I wouldn't do that in our bedroom or lounge.

Its actually quite easy to use wallpaper (vinyl seem to work best) on outlet/switch covers when you want something snazzy or for an accent. One has to play around with the "glue" a bit to see what sticks best and take a couple of trial runs on cutting the correct shape to be able to fold under the edges but the wallpaper sticks quite well and seems to last forever.

The biggest issue is who wants to buy a whole roll of wallpaper for just a couple of switch covers...

Use white outlet plates. As far as I'm concerned you want innocuous cover plates and not a design statement. Unless you have walls that are so dark that light-colored cover plates would be jarring, stick with simple. Having colored wall plates means having colored switches and receptacles as well, and then there is a lot to change out when a room gets re-painted a significantly different color.