The basic premise of the game is that each level is an island which is slowly sinking into the sea. The player must therefore get to the top of each level, by jumping on platforms and making use of the game's most distinctive feature: the ability to produce solid rainbows, which are used both as platforms to stand on, and as a weapon (remember the case in Bubble Bobble when using gum bubbles for the same reason!).
While initially appearing to be quite a simple game, Rainbow Islands in fact has a vast number of secrets for the player to discover, including secret levels, secret power-ups and riddles. Because of these elements, completing the game properly requires a large amount of dedication from you. In fact it is hard to master the controls because of badly programmed sprite animation found on the Amstrad CPC and ZX Spectrum ports...
The CPC version features excellent cartoon-style graphics, with lots of colors and variety of sprites. Most of the original (arcades) graphics details are here which makes the game really pleasant to watch with colorful and varied backgrounds. Although the cute sprites, it is hard to control your little hero as a result of probably bad programming in this aspect of the game. Sprites do not move as smooth as it should (in contrast to the Commodore 64 version which offers fluid sprite animation. More on that, screen scrolling (which goes always bottom-up) is not that fluid but this is something common due to CPC's hardware limitation (in contrast to the Commodore 64 version which runs smoother here).
Sound is great, offering the superb original in-game soundtrack along with a few nice sound effects.