Is it real brass?
If they are brass and look like real brass chances are they have a lacquer on it. The reason I say this is that brass will tarnish fairly quickly and can turn black. What keeps the brass from tarnishing so fast(and keeps it shiny and gold-ish) is the lacquer which keeps the air from tarnishing. Eventually (takes years), that brass shine will tarnish even underneath the lacquer though. The way to deal with that is stripping the lacquer(or you could just start sanding)... then shine up with a brass cleaner once that lacquer is off, and then re-lacquer.
Sometimes something will look like it is solid brass but is actually brass plated. I think you could still try the above method though, and you will eventually figure out what metal is all the way through. In either case, to reiterate, you could try shining it up with a brass cleaner, if nothing happens to make them look better, you will need to strip the lacquer to get to the brass like I explained above. Then try to brass cleaner it again. (TRY BRASSO cleaner) If still nothing looks better, then the metal is probably a cheap metal that has been painted to look like brass and can just be repainted and finished off with another lacquer or polycrilic finisher.
Here's a post that might help - https://www.diynetwork.com/how-to/maintenance-and-repair/cleaning/how-to-clean-brass#:~:text=Rub%20ketchup%20onto%20item%20with,warm%20water%20and%20dry%20thoroughly.
If you are sure it is real brass, use Brasso.
Dissolve a teaspoon of salt in 1/2 cup of vinegar. Add flour to make a paste and rub on the brass. Leave it to work for 10-15 minutes, wipe off and buff.
This post might also be helpful
Barkeepers friend also works. To keep brass from tarnishing can spray on a clear coat to keep air out that causes the tarnish.
Hello. They are dedicated brass cleaners but I find bar keepers friend to be quite an extraordinary cleanser to have on hand for multiple surfaces.
here’s a link that might be helpful.
I've always used brasso but I've heard of people using ketchup. I tried it on real silver and it removed tarnish fairly well.
Use Brasso or Metal Polish or Brown sauce should do it too.... Good luck!
I have heard that you can use catsup to clean brass. Give it a try, you probably have some at home.
You need to use Brasso!