Getting paint out of stone is very difficult.
The restorer has a real problem. If sufficient detergents and scrubbing are applied to remove the paint, it will also remove the patina, which is part of the stone and develops over hundreds of years. The surface of the stone reacts with the environment and changes gradually to a hydrated layer mixed with environmental dust and soot, and biological material. If this layer is removed, stone has been removed.
And once the patina has been removed from one area, that area will no longer match. The restorer then has to choose between leaving the unmatched area, or applying a false patina to blend it in. But then you are painting Stonehenge! Surely that can’t be the right thing to do.
Bttt
Druids must have had something to do with climate, or they died out like Shakers.