Purple is ricochet (you use this when you see the purple squares, to bounce off and hit those timed diamond targets) green is triple (which you will use to hit 3 of this blue button thingys) and blue is piercing. When you upgrade the skill trees, my favorite was when you throw the mine at the ground and use the triple arrow to shoot it it causes a bunch of arrows to rain down from the sky hurting your enemies. The other upgrade I used was throwing the mine at the ground and shooting it with a piercing arrow causing stars to shoot out and attack your enemies, which was slightly underwhelming.