Blasters are powered by two things: power packs and various gases. The power packs energize the various gases into a tangible bolt that is fired from the gun.
Therefore, it might have something to do with what kinds of gases are used...
Colors have no effect on how strong the laser is or what makes the guns tick or what the gun is used for.
Anyways, colors.
Green- In the PT, all the blasters used by Padme and co. on Naboo fire green bolts, as do their ships. In the OT, the green lasers are a signature thing of Imperial spacecraft.
Red/Orange- The universal blaster color, I think. In the PT, most things Confederacy of Independent Systems fire red bolts, droids and spacecraft alike. Jango shoots red. In the OT, the Rebels are the ones with all the red, though the stormtroopers also fire red bolts. The red/orange thing is just because they all kinda blend together anyways...
Blue- I don't think it's used in the OT, except for TIE fighter shooting miscolorations. It's only used by the clonetroopers in the PT.
Purple- The Geonosian starfighters were the only case where the shots fired purple.