I would paint it the wall color — truly it will not be noticeable if there is art, furniture and other decor in the room

We need context but usually you at least paint them the same color as the wall and if you show us exactly where this is we can help maybe a bit more. Poat a pic of the space showing the vent post here in a comment do not start another post.

Painting it the same color as the wall will help a lot. If you still find it troublesome you could get a decorative vent cover that is made to stand out and look like an intentional design element. We replaced an ugly standard vent cover in our foyer with a gorgeous vintage iron one and it became a beautiful statement people that people swooned over. Not sure what your style is but there are quite a variety of choices. Here are just a few. They come in different finishes and some are paintable.

Oil Rubbed Bronze Rockwell Plated Steel Craftsman Floor Register, 6"x10" · More Info

Heat Air Vent Cover Grille Cast Victorian Rose Thorne Overall 13 7/8" x 15 7/8" · More Info

Air Vent Decorative Cover, Black, 10" X 10", Aria, With Mounting Screws · More Info

Painting it the wall color is always a good first step.

There are several companies that sell more attractive covers.

The vent looks like it has peeling paint already. Before painting, clean thoroughly, sand and prime. Primer is available in a spray can. Or, you can purchase a new vent cover.