If you recall, Lucas originally only wanted Blue and Red. Had to make Luke's second lightsaber green because of how you could barely see the blue saber in the Tatooine shots.
As for an in-universe reason Sam Jackson has a purple lightsaber...Because he's the cat that won't cop out when there's danger all about?
What was YOUR ME3 ending: Merge with Helios, Illuminati, or Tracer Tong?
"I feel like I just watched Dwayne Johnson beat a small disabled child. Except the disabled child is really an infant clone of Hitler. It's so painful to watch...but so glorious." - Cheapy
"The Rebels only have one rank: traitor." - Eothain