The occasional green sheen of sandwich meat is no mystery, according to Jane Ann Boles [pictured here, demonstrating meat slicing for her students] and Ronald Pegg. They wrote a guide called “Meat Color”, for the Montana State University and Saskatchewan Food Product Innovation program:
Iridescence in Processed Meat Products
Iridescence is a common problem in sliced roast beef and ham products. The dominant color is frequently green and consumers sometimes confuse this with green myoglobin pigments associated with microbial growth. [This] is produced by a combination of the angle of incidence of the light on the muscle fibres and the wetness of the surface. If the fibres are pulled slightly out of alignment during slicing, the light strikes the fibre at an angle scattering light which appears as the rainbow or greenish color on the surface of the meat.