Found this article in the Los Angeles Times related on the topic you brought up….
"On Sticking Out Your Tongue
VOICES: A FORUM FOR COMMUNITY ISSUES
In the movie, "Seven Years in Tibet," the Brad Pitt character encounters a group of Tibetans who all at once stick out their tongues at him. No explanation is offered, but this custom has a long history.
A 9th century Tibetan king, Lang Darma, known for his cruelty, had a black tongue. As Buddhists, Tibetans believe in reincarnation, and they feared that this mean king would be reincarnated. Consequently, for centuries Tibetans have greeted one another by sticking out their tongues demonstrating that they do not have black tongues, that they are not guilty of evil deeds, that they are not incarnations of the malevolent king.
Nowadays, when Tibetans meet, they briefly extend their tongues as a greeting. They do not extend their tongues as far out or for so long a time as shown in the film. They would never extend their tongues as a group, either. In addition to being a greeting, sticking out one's tongue is used to show agreement, and over the past few years this gesture has evolved into a sign of respect.
Another custom incorporated into the film was the presentation of white, gauzy, rectangular scarves known as khata. Due to harshness of weather, Tibetans use scarves instead of flowers as offerings for prayers with special wishes. They offer scarves to priests for long life, to wedding couples for good fortune, to travelers for a safe journey, to new babies for protection."
Guess is true about sticking the tongue out.