There are 3 types of planets, each with a different color. The Red ones, the Green ones and the Blue ones. Each planet belongs to an "Orbit", just like the planets in our Solar System or like the Electrons that belongs to a Orbit in an Atom.
The first orbit is the Red, the second is the Green and the last orbit is the Blue. So you have to take the Red Planets first, then the Green Planets and the the Blue Planets.