Yes, you read that right: green skin. It started, I believe, with Mme de Staël's memoirs, in which she says that he had greenish veins. This, I suppose was taken to mean that he was entirely green. Or so Carlyle must have surmised by the time he wrote his "history" (the term is loosely understood) of the Revolution, in which Robespierre is referred to as the "sea-green Incorruptible."