BIC's 4-Colour Multipen ballpoint is a classic. You get 4 colours - black, blue, red and green - in one pen. 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 Mini version is perfect - big enough to write with comfortably, but at only 120mm long - 25mm shorter than the standard multipen - it will fit nicely in a pocket or small bag.