The colors of the doors might have some special meaning. Maybe something to do with the previous balls or something else entirely. Regardless, something about the purple door intrigues you. Maybe it's because purple is a nice color, maybe because it's a color that's unique so far or maybe just due to it being possibly a "neutral" choice.
Having made up your mind, you reach for the handle. Before you even touch the handle, the door swings open and sucks you in. Turning around you only notice a purple wall. You seem to be in an enclosed room that's entirely purple and illuminated by an unknown source. The square room offers nothing except a spike in the middle and there doesn't seem to be a way out.
Is a sacrifice required?