Rainbow Brick

Rainbow Brick is a Hardmode brick that can only be obtained as a drop from the Rainbow Slime. They cycle through the colors of the rainbow. The cycling colors of Rainbow Brick Walls, the Disco Ball and Rainbow Hair Dye are synchronized with each other. When a waterfall passes over a Rainbow Brick, said 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).
 * If you let water pass through rainbow brick by turning the brick into a half slab, it will cause the water to change color with it.