Have you tried a dark bronze like the other metal elements in your room? I grew up in a house from 1906, and the registers were all cast iron and each was different in different parts of the house. A simple grid was in the living room. I think anything goes in a house that old, that looks good. There doubt there was ducted heat back then, right? So there weren't registers. But that is a guess.

Old registers I found on the web.

Here is a link that might be useful: Signature Hardware grate

Good news is that there are nice looking registers out there that are very reasonably priced. Even four years ago, when I was looking they were hard to find. I think you could do white, chrome or dark to match your other hardware. If you find the one you love and it's not the right finish, you can always paint. I have a small bottle of 'Sophisticated Finish' that I have used to paint quite an array of objects. I can't believe how far it has gone.

Anyhow, I've seen nice ones at Costco in 3-packs, and this one at HD I think would go very well with your floor.

Here is a link that might be useful: floor register

Signature Hardware has a whole pile of them to pick from.

-Babka

Here is a link that might be useful: Signature Hardware floor registers

Just to be clear about my posted picture of old registers. It wasn't a suggestion that you use one of these old ones. It was an illustration to show a precedence of style, from the earlier 20th century. I would only get a new one, but probably in the style of old, similar to the links for new ones we all provided.

Or try Pacific Registers. I'm thinking about a cast iron "look" for two wall registers.