Michigan State University Extension
Preserving Food Safely - 01600601
Because both acid and heat are destructive to chlorophyll, a bright green color is not to be expected in pickles. During pickling, the magnesium atom in the center of the chlorophyll molecule is replaced by two hydrogen atoms to form pheophytin, an olive green pigment. While the olive green color of pickles is not as attractive as the natural green color, it is accepted as characteristic of pickles.
If copper replaces magnesium in the chlorophyll molecule, the pigment takes on a vivid green color. Formerly copper kettles were used for cooking pickles in order to impart a bright green color, but this practice is not recommended. Some old-fashioned pickle recipes call for using a "blue stone"--namely copper sulfate-- to give the pickles a bright green color. We do not recommend this practice because copper can be toxic if present in large enough quantities.