Nice little game but boring.
You can add some things to make it more interesting. Have the different colored enemies have different behaviours/attacks.
You can make them move in patterns, shoot at you, charge at you, etc. Also add different guns: pistol (default), shotgun, machinegun, etc. Keep the cooldown bar on the right, I like it.
Make use of the death animation. For example, each enemy of a given color explodes with the same color explosion. All enemies of the same color near the explosion also get killed. This way you can try to chain enemies together to make them explode. This way you will be combining that match 3 game with a top-down shooter.
Keep up the good work!