What you do is this,Go to the local grocery store.
Get your self some of the cheap store versions of kool-aid.
But some dry ice (not all grocery stores have this, but many doo)
Put your water in the 1 liter bottle, add a little of the store version of the kool-aid to color it.
Add a small piece of dry ice. You will have to play with this; you want enough dry ice to increase the pressure but not bust it by its self.
This way, you have a colorful liquid in a container with high pressure and ready to go boom, and when you shoot it with the 22 caliber rifle, it will "explode" due to the high pressure inside releasing a show of color!
All very cheap and easy, and relatively safe.