Is there a product to change color of metal?

Yes, there are off-the-shelf products (like these ) to change the color of copper alloys, or you can mix your own for specific effects.

You’d need to make sure the metal is absolutely bare, with no protective finish on it. Most brass gets a coat of some kind of varnish at the factory to keep it from oxidizing, and that would have to be completely stripped off before applying the patina.

Metallic spray paints work well. Here is an article telling how to do it.

I used this product to cover some brass hardware on a piece of furniture and was happy with the results. I would be concerned, however, about the effects of heat on the product if you use your fireplace. A fireplace screen is probably going to get pretty hot. (From Micheal’s Crafts)

@syz We don’t use it at all, so that isn’t an issue – but thank you for pointing it out. I would hate to finally use it one day and poison my family or something.

Brasso