Structural colored balloons (SCBs) composed of poly(4-vinylpyridine-co-styrene) (P4VP-PS) exhibited a pH-controlled structural color change in the presence and absence of p-toluenesulfonic acid. The diameter of the SCBs increased and decreased under acidic and neutral conditions, respectively. The different colors exhibited at different pH values supposedly resulted from a change in the shell thickness not only due to the change in the diameter of the SCBs but also due to the uptake of p-toluenesulfonic acid to the pyridyl side chain of P4VP-PS.