Paint on old hardware can be removed by placing the old hardware in a pot of water and taking it to a low boil. This softens the old paint and makes it possible to remove it with some elbow grease, and a plastic scrapper.
Be careful when removing the hardware from the water it will be extremely hot to the touch. I recommend wearing gloves and removing the hardware from the pot with old tongs.
Whatever you use to boil the hardware in or to remove it from the pot I would not use for cooking purposes ever again.
You may very well have to "dip" your hardware back into the pot and reheat it a few times before you get all the old layers of paint off.
I would not use this method for old glass or cermaic knobs as the heat might crack them BUT for metals you should have no problem.
I've used it on door hinges and even some antique window hardware that had been painted over several times.

I've also stripped all of our hinges and door plates by either using liquid stripper or the boiling technique. I follow up with a nice wire brush to knock out any paint in small areas.

Not sure what your fixtures are made of but VanDykes's carries a polishing agent that darkens brass.

http://www.vandykes.com/

What are your fixtures made of?

I used Safer paint stripper and a dental tool on my letter slot. It worked better than anything else I tried. My neighbor suggested boiling to get the last bits of paint off. Well, in my case that was a mistake because the letter slot was made of "pig metal," a cheap kind of iron, and the boiling made it rust like crazy. So, boiling can be good, but try to guess at the underlying metal first. I lost some crispness of detail on my letter slot because of the boiling.

I do the same thing for my old hardware. I also add a small amount of Downey and an even smaller amount (1/2 tsp.) of Tide (any detergent). This helps to loosen the paint while you are slow boiling it.

I then take a soft bristle brush (I have found that wire scratches the patina) and oil to remove any remaining paint residue.

As for changing the color, I think your only option is to spray paint the hardware.

Painting hardware must have been in fashion at some time because majority of the hardware in my home is painted to "look" like brass. Honestly, I don't recommend it as over time it chips and flakes off and looks awful.

Crock Pot for 8h. I used a bowl within the Crock Pot bowl so I wouldn't get the Crock Pot "contaminated".
Paint came off the hinges fairly easily.

"Paint on old hardware can be removed by placing the old hardware in a pot of water and taking it to a low boil. "

Just make sure the hardware is not steel (or even worse partly steel and other metals).

It will rust heavily doing this, and if more than one metal is present the corrosion can be pretty bad.