To get this trophy you need to pop 15 critters at once. Critters added by combos (which count as separate chains) will not add to this total. While this is probably possible to do early on, it is not advisable to aim for it. Trying to build such a long chain of only one color, going for this will probably lead to level failure and slow down the rest of your progress in the game.
This will be much easier to do using the wildcard critters or once the paint powerfood shows up in the game. Chains extended due to wild card critters (even multiple) WILL count towards this trophy. Chances are, you will get this trophy without specifically going for it later in the game once wildcards and paint are involved.
If you don't get this trophy through normal adventure mode play, try Challenge level 37. The extreme number of wildcard critters that spawn on this level will make it almost automatic to get a chain size of 15. Challenge level 37 won't unlock until you are almost 3/4th of the way through the adventure mode.