Now THIS is art of a very high caliber, indeed!
It was just a public visual detail that elicited a stupid response from the very stupid people, and probably some delight from the rest of the population.

If I was one of the chief stassi goons in town, my response would have been “counter-intelligence art” or “counter-art”, painting MORE stones purple and even other colors, so that whatever secret message the original ones were conveying would be confused, drowned out.