No, silly, not painted green! Nor itty bitty little churches where itty bitty little people* go to worship.
No, these churches are massive edifices to the highest and best culture and architecture could provide at the time – cathedrals.
Cathedrals were filled with stained glass windows, and pictures of the cross and other holy objects frequently were coated with very fine gold. So fine, in fact, that it was tantamount to be nano-size particles. And these, when hit by the sun, vibrated in particular frequencies creating magnetic fields that actually purified the air of volatile and toxic chemicals.
And, as far as we know as of today, these stained glass windows coated with gold were the first examples of nano-technology at work.
Interested? Read more in this article from CleanTechnica.
* Gratuitous reference to The Fifth Element, where Zorn discusses with the priest the facts life from his perspective and a manufacturing god.