Pixie Dye

Strange Plant turn-ins hand you one random dye out of a large pool, so this is something you collect toward rather than target. You gather plants, cash them in, and take whatever the trader feels like giving you that day.

There is a second lock on top of that: it will not appear in the reward pool at all until Plantera is dead. The effect is a warm yellow-orange wash close to the light Pixies give off, though it is only a look. Nothing you dye with it actually lights up a room.