Rainbow Brick

Rainbow Brick is a Hardmode brick that can only be obtained as a drop from the Rainbow Slime. They cycle through various colors. The cycling colors are synchronized with each other, Rainbow Brick Walls, the Disco Ball and the Rainbow Hair Dye. When a waterfall passes over a Rainbow Brick, the waterfall will also cycle through colors.

Trivia

 * Next to Lihzahrd Brick, Rainbow Bricks are one of the hardest blocks to obtain, being that they never occur in the environment, cannot be crafted, and only drop from Rainbow Slimes -- which only spawn during Rain or Blizzards near the Hallow. Despite this, Rainbow Bricks still only have a rarity of white (second lowest only behind gray which only applies to trash items and white tier tools with negative modifiers).