John Doom said:

Anyway, I don't like the "RGB color scheme" either, but I think it would be funny to have lightsabers of whatever color too. I liked KOTOR's idea that every color is associated with a specific jedi role, but it annoys me that the sith (why do they even exists!?) are supposed to wield red lightsabers.

Well as far as the EU is concerned there is an explanation for that. Apparently red crystals are more compatible with dark side energy, as well as being synthetic which is due to the Jedi having exclusive access to the planet were lightsaber crystals are naturally occurring. And as you said all the jedi colours have since been given different roles and hierarchies.
Over all it's like all Star Wars these days, way to caught up in it's own lore. In the first two films it was simply. Good guys = blue, evil guys = red. And then they threw in green to add some variation. The fans got really anal about why it was green, so the EU came up with an excuse for it.

Similarly we have the blaster bolts which have since the very beginning been in practically all the colours of the rainbow, for no other reason than to look cool. Of course the EU/fans have come up with some explanations there as well.

EU (and fan) explanation of lightsaber colours:
https://www.youtube.com/watch?v=qTKaWcpyBdY

Fan explanation for blaster bolt colours:
https://www.youtube.com/watch?v=jifGMZBz65k