They're basically color-coded according to your beam weapons. The power beam pirate (the yellow guy) is, in my opinion, the most dangerous because his shots seem to do the most damage, like DRJ said, one or two super missiles are the best way to take care of them. The ice pirates are easiest, just tag em with the ice beam, and launch a missile to shatter them, just like a metroid. The pink (should be red....plasma) pirates catch on fire if you use a charged plasma shot, which will help kill them faster. As for the purple Wave Pirates, keep firing charged wave beam shots, as it keeps them incapacitated. Don't, however, bother using the wave beam combo, as it will waste missiles that you don't need to use on him anyway.