Mixed with lime, copperas is a traditional way of coloring lime plaster yellow. Its distinct and unique non uniform appearance makes it easy to spot. Copperas is a greenish crystalline compound (ferrous sulfate or FeSO4 • 7H2O), which is also known as green vitriol or iron vitriol. A wall colored this way, will change with climate and darken and intensify over time, adding an extra dimension to the term "a vivid color."