Is there any truth to Drug Recognition Expert claims that marijuana leaves a green coating on a person's tongue?

Part of my job involves reading police reports, and I came across a DRE claiming that he identified that a person had likely smoked weed because of a green coating on the the tongue. Is there any truth to this? I did a bit of googling and this seems to be a common police training clue for marijuana. All the pro cops cite it as solid investigation and all the criminal defense attorneys claim it is bunk, whats the real science going on here?