How to craft it
Sky Millbuilding with it
Cloud blocks already give you that soft, weightless surface for sky builds, and this version adds a slow color cycle plus enough glow to read a room by. That makes it useful for floors and ceilings where you want light without hanging a lamp every few tiles.
The color shift never stops, so a whole interior of it gets noisy fast. A border, a stripe of trim, or a lit walkway reads far better than a solid wall. Normal worlds never generate it, but the Rainbow Road seed swaps the sky islands over to it, which is the cheapest way to see how it looks in bulk before you spend a stack of Rainbow Brick making your own.