Purple is the color of the expansion symbol of the timeshifted cards in Time Spiral. Because they appeared in a rarity significantly different from any other rarity, they were assigned their own new rarity level (higher than rare).
Ever since a prank article in the InQuest #22 (February 1997) which featured purple cards (see Gallery), purple has been speculated to be the unreleased sixth color of Magic.
For Planar Chaos R&D actually playtested purple as a sixth color in the color pie, to convey the alternate reality of the set. A purple version of Mana Drain was planned and the basic land would've been either City or Cave.