In the books only blademasters got purple blades. They were the only ones to master all 7 forms of lightsaber combat.  Samuel L. Jackson didn't know that though, he just wanted a purple lightsaber so he could find himself easier in that big lightsaber battle.  I lost some respect for Jackson because of that, but I love that Star Wars nerds have  decided that he was a blademaster and not just a vain actor.
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