What were the emerald city horses covered in?

The horses in the Emerald City in L. Frank Baum's "The Wonderful Wizard of Oz" are covered in emerald green paint.

This is mentioned in the book:

"The horses were all a beautiful green, and the men who rode them wore green uniforms and green caps."

This green paint is part of the overall theme of the Emerald City, where everything is made to appear green due to special spectacles worn by its inhabitants.