There is no scientific support for this claim. There is nothing to suggest that recent marijuana use and a green tongue are related. In fact, it seem counter-intuitive as the plant has been dried and burned before it is consumed.
So the next time a law enforcement officer tries to bamboozle the court with the green tongue argument, counter with the unreliable and unscientifically proven argument. Find the Motion in the Library and convince your judge that this unreliable evidence should not be admissible.