What is Rainbow Road?

Rainbow Road requires you to clear a level on the first try.  When you complete a level, Kimmy will appear on the Rainbow Road showing the levels that you need to clear.  As you can see by this print screen, you would be ready to play level 1798 since you just cleared level 1797 on the first try.  She is now going to add the color bomb to level 1798.  If you clear this level on the first try, the map will open up again and level 1798 will now turn green.  You can see that your rewards increase and they will continue to increase as long as you keep clearing each level without losing a life. However, it you fail a level at any time, you will lose the boosters in current level.  When you do clear the level, the Rainbow Road map will open up and you can then view what rewards will show up on your next level.