Copper sulfate is one material that produces a green flame when burning. Using alcohol as a fuel source can result in a brighter flame due to the fact that alcohol burns blue.
Other materials that result in a green flame include boric acid, borax, barium compounds and ammonium compounds. While these materials consistently result in a green flame when added to a fuel source, impurities can add other colors. For example, the sodium in a typical camp fire produces the familiar yellow flame when burned, which can overpower and outshine the green. These materials are not safe for use in cooking.