Lots of ways to go about this.
If you go back to my post "Cool Things to Make You Think", (I think it's in the November archive) you'll see a cool dangly piece that is actually plated brass, but it's been painted on with pearly pastel paint pens, like you'll find in the crafts store.
I like Zig Painty Markers, but there are lots of other brands that work great too, if you want to pop a little color into a plated piece.
But RAW BRASS poses the best possibilities for a finish that's uniquely yours.
For years I've had success with a combination of mixed brown and black acrylics, painted on, wiped off, painted on, wiped off, and highlights tweaked into the piece with Topaz Brown Color Magic (stuff is in a bottle, you paint it on like nail polish.)
Some like permanent markers. They color the piece and wipe off with a piece of paper towel or a flat rag (no lint) that's soaked up some acetone nail polish. Just watch out if you wear fake nails, acetone will ruin them!
Others have had success by spraying a combo of black and brown spray paint into a tumbler along with a few drops of paint thinner, adding their components and tumbling for several hours. Let the components dry on wax paper or newspaper (probably newspaper is best) til dry. You could also apply the paints FIRST, and THEN tumble. I honestly have not tried this but my gut says it should work very well, I hear good things about it from others. What I like about this, too, is that you should have very little exposure to solvents, just maybe put a respirator on, when you spray the paint into the tumbler.
Anytime I can avoid harsh lacquers or solvents, I'm all for that.
Another thing you can try is the buffer on a Dremel tool, or, if you have access to a big buffer, that's cool, too. I actually have a great big old bad buffer with a double wheel, in my workshop. I've used a soft buff with tripoli on raw brass and oh baby, it looks LOVELY. The yellow in the brass shines up just GORGEOUS, and you get alot of darkening into the crevices of the piece to bring out detail. So you might like to try that.
Others like Liver of Sulfur. Warm your components first; you might try just using a blow dryer to do that. Then dunk 'em in liver of sulfur solution per the instructions on the package (you can get it from a lapidary, or just Google it, not hard to find a supplier, this stuff is not uncommon) til you get the effect you want. Let them dry. You can buff in some highlights with a buffer.
There are other chemical dips you can buy for brass AND copper, most of them require care in use as they are a bit noxious. Remember, anything you get on your skin isn't that much different than if you ingest it; it will get into your system. People forget that. So if you decide to use chemical agers on brass, FOLLOW THE DIRECTIONS.....KEEP IT OFF!!! YOUR SKIN.....and DON'T WORK WITH IT TOO OFTEN.
There are some cool books out there on the subject of patina on metal, I keep reminding myself to buy them but have given them pause more than once when I visited Barnes and Noble. If you're into learning more, check out Barnes and Noble online, or visit Amazonbooks.com. ......you know them, they have everything.
Be fun! Check out my raw brass stampings at http://www.bsueboutiques.com, and BE SAFE.