BIC's 4-Colour Multipen ballpoint is a classic. You get 4 colours - in this case light green, black, blue and red - 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 one will be hard to lose, with its fluorescent green translucent barrel.