So if there are blocks with colors "red" and "green" on the screen, but no "blue" block, you need to spawn a bullet and set its color to randomly to "red" or "green" (but not "blue"). Is this correct?
You can do this:
System Pick random instance of Block
-> Create Bullet
-> set Bullet animation frame to Block.AnimationFrame
The problem with this method is that if you have 9 red blocks and 1 green block on the screen, then there is a 90% chance that your bullet will be red.
Also, note that if you are doing this inside some parent event (for example "Bullet on collision with Block"), you will need to execute "System-Pick all Block" event first.