After completing level 12 you will unlock the Color Bomb booster (image 1) and level 13 is the first level in which you can use this booster (image 2). By activating the Color Bomb booster you will start the level with a Color Bomb candy on the playing field.
With candies of 6 colors on the playing field it can be difficult to match candies an create special candies. This doesn’t make a difference for the Jelly in the middle of the playing field but because it tihs it can be difficult to clear the Jelly at the top of the playing field. So when you get the chance to clear Jelly at the top of the playing field, don’t hesitate to do so.
During Jelly levels you should match candies as low on the playing field as possible as often as you can. Doing this will shift the largest amount of candies, hopefully matching some automatically and create new opportunities to make special candies.