A green tongue indicates that a person is suffering from a condition called black hairy tongue. The condition is commonly characterized by black color, but it may be green in other people, as stated by WebMD. It can also be brown or yellow.
This condition may develop because of poor dental hygiene, dehydration, smoking tobacco, using mouthwash that contains menthol or peroxide, drinking a lot of coffee, using antibiotics and low amount of saliva. This condition is common in HIV patients and those who use intravenous drugs. Other than the color, the condition does not have any other symptoms or discomforts.