BIC's 4-Colour Multipen ballpoint is a classic. You get 4 colours - in this case, violet, pink, orange and yellow - 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.

The 'Sun' version has a cheerful sunny yellow barrel.