At the end of a section of the first level of Vegetable Valley, there's a warp star. You're not supposed to go on forwards, because there is an invisible wall blocking Kirby, but backtrack a little bit behind the cutter enemy (Sir Kibbles) and go back. The warp star despawned, so Kirby can go even further to the right. Then, spelled in blocks, is the word HAL.
If you take the first letter from every area it will spell out VIBGYOR. Read backwards is ROYGBIV, an acronym for remembering the colors of the rainbow.