The game requires a green bullet, a blue bullet and a red bullet. To get a strong overall gun, obviously look at the power output of each bullet per color. Additional bonuses comes from the elements (2 same elements additional 1 more friendship) and the number of stars on each bullet
On the top left corner of your bullet, you can see the bullet's element. If 2 or 3 bullets share the same elements you willl gain bonus friendships