Rainbow Brick

The block cannot decide what it looks like. Sometimes it places with mortar lines like an ordinary brick, sometimes as a flat unbroken face, and hammering it can flip it from one to the other. Getting a large wall to read consistently means placing, checking, and reworking sections by hand, which is a poor use of a material this hard to get.

It cycles through the spectrum on its own and every other rainbow object in the game cycles with it. Rainbow torches, rainbow walls, a disco ball and the matching hair dye all shift in step, so a room built out of the lot reads as one effect rather than several competing ones. Paint does not stop the cycling. Water running over it takes the color as well, though lava, honey and shimmer ignore it.

Rainbow Slimes are the only source. It cannot be crafted and does not generate in ordinary worlds, so anything larger than trim work is a farming project first and a build second. Slime rain and a slime statue setup in hardmode are the usual answers. It also feeds a few later recipes, so hold some back rather than pouring your whole supply into one floor.