Rainbow Slime

The Rainbow Slime is a hardmode monster that spawns in The Hallow. It can only spawn while it is raining, or during a blizzard if you're in a hallowed Snow Biome. Unlike other Hallow enemies, it does not spawn directly on hallowed blocks, but instead spawns on non-hallowed blocks such as normal grass or snow. These conditions make it a somewhat rare monster, though it's easy to encounter if the conditions are right. Its color shifts constantly, as do the Rainbow Bricks it drops.

Tips

 * Rainbow Slimes tend to spawn more frequently in hallowed snow biomes than in forest biomes because there are fewer hallowed blocks exposed on the surface of a snow biome. If you want to find Rainbow Slimes in a hallowed forest, you can set up a Water Candle near the edge of the hallowed biome and wait there while it rains. Make sure you are standing in the hallowed biome (with the Hallow background) while allowing enemies to spawn off screen on non-hallowed blocks.
 * Alternatively, you can cover hallowed blocks with snow or bricks of some kind to allow Rainbow Slimes to spawn. Note that this will also prevent other Hallow enemies from spawning.