I use a Lamy 2000:
http://www.rickconner.net/penspotters/lamy.2000.html
Specifically, I have a four color ball point version. You don't twist it, you just hold it so the color you want is facing up. I replaced the green cartridge with a Lamy stylus refill.

I think it's got a really nice, clean design, and is one of the best looking multi-color ballpoint pens out there. It looks modern, despite having actually been designed in 1966.

The tip is grey rather than the bright orange that some of the others are, but personally I do prefer that.