Source For Green Or Black "Carbon Spots"

Possibly true, but unlikely that it is for every coin with such spots.

"Carbon spots" are caused by droplets of something landing on a coin. Very often, that "something" is indeed saliva, particularly from someone sneezing, coughing or talking over their coins. Saliva spots will tend to go black on silver and copper coins because saliva contains sulfur-based enzymes; these sulfur compounds react with the metal as they degrade. Saliva is pH neutral; the enzymes in your mouth that digest starches won't work under acidic conditions. Another common cause of "carbon spots" are droplets of oil.

Green spots are usually inorganic in origin; they may be caused by a drop of something acidic or alkaline landing on them, but if your saliva's pH is that abnormal, you'd better go see a doctor... or move to another planet, where the food is likely to be more edible for you. But salt (from, for example, ocean spray) or some other aerosolised pollutant is also likely to be a causal agent of copper coins breaking out in green spots.