This paper deals with the characterization of a green paint layer covering ornamental stone elements in the Presbytery of Arcos de la Frontera's Chapel of Mercy (Cádiz, Spain), a 16th-century building. Optical microscopy, X-ray diffraction, Fourier-transformed infrared spectroscopy and scanning electron microscopy were used to determine that the layer derived from a treatment of the stone (calcarenite) consisting of applying mixtures of gypsum and zinc-copper basic chlorides as paint pigments. Among the different salts, zinc-stabilized paratacamite, clinoatacamite and atacamite could be present. Very scattered dark-maroon speckles appear on the green layer (mainly composed of cuprite). Both these salts and other greenish, synthetic copper-containing compounds may be products deriving from artificial corrosion of brass or other copper alloys.