They've been inconsistent in recent years - the Soul Gem was purple in Hickman's New Avengers, for instance, despite always having been green on Adam Warlock's head.
"Classically", purple is Space, blue is Mind, and pink is Power - while Reality is yellow, Time is orange and Soul is green. (Although, TBF, back when they were just the "Soul Gems" pre-Thanos Quest/Infinity Gauntlet, they were ALL green!)