Green serpent - temptation by alcohol.
The expression owes its origin to the traditional Russian designation of an alcoholic drink - "green wine" (according to the color of the grapes from which it was made). The adjective "green" began to gradually acquire a sharply negative assessment, since drunkenness, as you know, is a vice. Let us also recall the biblical serpent-tempter: at the junction of the meanings of these two expressions, the phraseologism "green serpent" appeared - the temptation by alcohol.
source - gramma.ru
I don't know about the "green snake", but here's why I can tell you about the "bruises". I read about bruises in the magazine Dermatovenereology for 1977. There was an article called "The Blue Man," which described the effects of ingesting wood stain that was sold in hardware stores. The stain was made with ethyl alcohol, and the coloring pigment was methyl nigrosine. After using this stain, persistent staining of human tissues, including skin, in blue color occurred. The staining lasted up to 6 months, then it passed.