Well as i understand it jedi's use certain crystals that make the sabers either blue or green.
Sith's use sith crystals and it makes their sabers red.
sith crystals make the sabers more powerfull(not much) but not as agile as the jedi crystals.
Yellow sabers? never seen them
Mace has a purple one just cause he requested it to Lucas. Also it says its an electrum lightsaber. Mabe its purple cause it doesnt use crystals its electric(!?).
All my info comes mainly from the visual dictionary