BIC's 4-Colour Multipen ballpoint is a classic. You get 4 colours - black, blue, red and green - in one pen and it's refillable too. All you need do is push down the side button for the colour you want, and retract it by pushing down any of the other buttons. It has a handy pocket clip and a little ring on the end, just in case you need to attach it to something.

This version has a nice grippy grip area, to stop your fingers sliding up and down.