dop2000 is correct when he says it's probably better to handle the bullet with one object rather than three, presuming the bullets all behave the same otherwise. I like the idea by the way, only allowing the player to shoot matching enemies, that's cool.
If you need to do this still (ie, bullets of different colours behave very differently and can't be handled using a single object) you can do it using a variable on the player. Make it a text variable, and whenever the player fires a weapon (or on the start of the layout) you fire the currently indicated colour of bullet, then set the variable to:
choose( "red", "blue", "yellow" )[/code:3q68yvx4]