Rainbow Islands/Secret codes

Secret Rooms
If you collect each of the small diamonds on an island in order, either from left to right (red to violet), or right to left (violet to red), a silver door will appear at the top of the room with the boss. If this door is available, you can skip the fight against the boss and collect that island's big diamond behind the door. You will also be awarded a special item which usually grants you some special power that will last until the end of the game, even if you die. For even more points, defeat the boss as you normally would, collect the large diamond, and then enter the silver door before the game advances you to the next island, where you can collect a second copy of the same diamond for a higher score.

Secret Codes
At the top of each secret room are a series of letters. These letters are codes that you can enter on the title screen for a secret effect (during the animation of the rainbow). Each letter is an instruction that you must enter on the control panel. L, R, U, and D correspond to pressing left, right, up, or down on the joystick. J is for jump and B is for rainbow. S is for start (the 1 player button). Only one code may be in effect at any given time. When you insert a coin, an item corresponding to the last code used will appear in the bottom left corner of the screen. With the exception of the first three hint codes, the codes have all been changed in the Extra version of the game, and are much harder to discover (read the explanation below to learn more.)

Game Hints
There are six hints that the game provides, three hints in the regular version, as well as three new hints in the Extra version. To obtain the hints, you must either find the sceptres that are responsible for making them appear, or you may enter the secret codes on the title screen that reveal one hint.

Cracking the last two hints
The password provided in Hint F is actually the (Japanese) names of all the enemies from Bubble Bobble, with one letter missing. They break down as follows:

The answer to Hint F is "LASTDIA". In other words, the color of the last diamond that you collect on an island before entering the secret room will determine which of the seven remaining characters in the code will be shown. In order to get the complete code for each of the Extra secret codes, you must complete each stage seven times, collecting a different small diamond last before reaching the boss and entering the silver doors.