I think it was said earlier in here, that originally Lucas wanted all the good guys to have Blue sabers and all the bad guys to have red ones.
It was simply so you could go "He has a red saber, he's a bad guy".
Then Luke lost his saber in Episode V, and had to construct a new one. It came out green. But why was it green? Because a blue saber wouldn't show up good against the sky backdrop for the Jabba's barge fight thing. (Note: before this, all lightsaber use was restricted to indoor areas).
So, there you have it. Blue and Red were the only two lightsabers there were originally supposed to be, then green was added so we could see what Luke was doing with his saber.
Why was purple added for Mace Windu? Sam asked for one.
And the books have gone and used almost every color imaginable for sabers from topaz to cherry red and so forth (I know, red was mentioned before, for the movies).
As for the RGB saber select. It would be cool, especially since it would give everyone a more personalized saber type. However, when you break it down, all the different saber colors would be are just variations in brightness on what we have now.
P.S. I am aware my grammar sucks. It happens after a couple days with no sleep.