Next to Lihzahrd Brick, Rainbow Bricks are one of the hardest blocks to obtain, being that they generally never occur in the environment, cannot be crafted, and only drop from Rainbow Slimes. Despite this, Rainbow Bricks still only have a rarity of blue (third lowest, behind gray and white).
The aesthetic of the brick, when placed, will either have the rectangular appearance of a standard brick, such as a Gray Brick, or a uniform blank appearance, such as that of a Smooth Marble Block. It may also change between one and the other when hammered. This can make it a rather tedious building material to use.
Rainbow Bricks will still maintain their rainbow-glowing effect even when painted.