You can age polished brass with gun bluing liquid from a sporting goods store. You can also buy proprietary stuff for the same purpose, but I don't know whether that has any real advantage.
I was shown how to do this by a guy with an antique lamp restoration business. The brass should be clean -- as in alcohol-wash clean -- and handled only with forceps until the aging is done. It's easier to do with small pieces that you can dip. For long pieces, I guess you would brush or spray it on. As soon as the brass reaches the desired shade, you wash off the solution.
That convergence looks pretty good. If there are secret tricks for making it perfect, I'd love to learn them. I don't think any of my old color sets has ideal convergence.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html