There are many different types of planets in the Master of Orion galaxy and they appear in many different star systems.
The following are general rules. For each type of star system, the habitable planet is generated randomly within a certain range. Good worlds can be in the worst systems and bad worlds can be in the best ones, that is the nature of randomness. The best worlds do, however, tend to be in the best systems.
Also, keep in mind that even the worst systems are usually still better than no systems. Having the production of a hostile world with a low maximum population going to produce at least something for your empire is still better than not having it or having it generate production for some other civilization instead.
The idea is to fight over almost every single world you can get, but some concerns do override this, those can be discussed later. At the beginning, you just want to scout everything you can scout and colonize everything you can colonize as fast as you can safely colonize it.
Yellow[edit]
The best. These are the most likely to produce habitable worlds. The habitability of a star system increases slowly till it peaks and then declines slowly again. These stars are the ones that happen to be in the perfect conditions for habitability in the millenia during which the game takes place.
Green[edit]
Second best. These are either just short of their prime or just past it. They are close to optimal, but just not quite optimal. If the game had taken place in a different millenia these might have been yellows.
Red[edit]
One of the worst. These are well beyond their prime and are very often on the bad side of the spectrum in terms of habitability. Don't hope for too much when scouting these systems early or at all.
Blue[edit]
Another one of the worst at the beginning. These are way too young, generally, to have conditions conductive to habitability. They usually have very hostile environments and require very advanced technology to colonize. They do, however, tend to have higher chances of better mineral deposits which increases factory production on the world if it can be colonized.
White[edit]
Another one of the worst. These planets tend to be too close to the sun and are very hostile, being far too hot (inferno environments) or receiving too much radiation (radiated environments). It is highly unlikely these can be colonized early and usually require the highest colonization technology to colonize at all. They tend to offer no advantages over other better stars to offset this advantage as well.
Purple[edit]
The rarest type, only a few per galaxy will be this color. They have very good chances for mineral richness although the environments still usually require technology to allow their colonization. At best 4 or 5 worlds will be of this type per galaxy and they are highly coveted in the stages of the game where most of the easy worlds are taken and civilizations are trying to research the technology to gain access to the hostile planet environments.