Paintballs are basically a food product and must be kept at room temperature for best results. An extreme hot, cold, or excessively humid environment can make your paintball more bouncy or brittle. They are biodegradable and non-toxic, and dissolve away if left outside. This is why paintball have a shelf life of 1-2 years depending on the type of paintball. The higher end thinner shell paintballs have the shorter shelf life, and the thicker shell of the entry level or field paintballs have the longest shelf life.
The picture is only for your reference, the details are subject to the sample.