Paranormal Activity 4

On October 9, 2006, Kristi Rey and her husband Daniel are killed by her demon-possessed sister Katie, who then abducts Kristi's one-year-old son, Hunter. Text states that Katie and Hunter's whereabouts remained unknown.
In November 2011, Alex Nelson lives in suburban Henderson, Nevada with her father Doug, mother Holly, and little brother Wyatt. Alex and her boyfriend Ben discover their new neighbor's son, Robbie, hiding in their family treehouse. The next day, Robbie's mother falls ill and is taken to the hospital, leaving Robbie, who begins mentioning an invisible friend named Toby, in the care of the family. Over the next few days, many supernatural occurrences begin to take place.
The activity soon becomes violent; a chandelier falls from the ceiling and almost lands on Alex. One night, Alex notices several cars parked outside of Robbie's house and goes to investigate, but flees after being confronted by a strange woman. Later, Robbie draws a green symbol on Wyatt's back, which Alex later discovers is associated with witchcraft and demonic possession. Robbie takes Wyatt to his house and when Alex chases after him she meets Robbie's mother, revealed to be Katie. Then the knife that disappeared almost kills Doug. Wyatt later confides in Alex that Katie told him he was adopted, like Robbie, and that his birth family needs him. Later Wyatt, while taking a bath, is pulled underwater in the bathtub by an unseen force, he rises in an apparent trance. Later that night, Wyatt goes into Alex's room and levitates her while she sleeps. The next day, Wyatt refers to himself as "Hunter", telling Alex it is his true name.
While alone in the house with Wyatt, Alex video-chats with Ben about her parents being mad at her when she hears the garage door open by itself; upon entering she is locked inside. Katie appears in the house and goes upstairs to Wyatt's room, where she tells him that she'll wait until he is "ready". The family's car turns on by itself and begins spewing exhaust, but Alex escapes by breaking into the car and crashing through the garage door. The next day, Holly is violently thrown against the ceiling by the demon and dropped to the floor, killing her; Katie then appears and drags her dead body away. Ben arrives and leaves Alex a message before he is killed by Katie, who snaps his neck.
Alex and Doug arrive home and Doug goes next door, thinking he saw Holly and Wyatt. As he goes to investigate, Alex finds Ben's body before she is attacked. She flees to Katie's home, and searches for Doug but cannot find him. Alex eventually finds a screaming and helpless Doug being dragged down the hall to an undisclosed room. As she continues to search for Doug, she encounters the demonically possessed Katie who sprints towards the camera shrieking. She escapes Katie and finds Wyatt in the yard, who refuses to leave. When Alex looks up, she sees the coven approaching her; as she turns around a demonic Katie attacks her while the camera falls to the ground and the screen cuts to black.
In a post-credits scene, two Spanish-speaking men go into a mysterious store. After finding witchcraft products, they decide to leave but a woman encounters the two and says "esto es solo el comienzo" ("This is only the beginning"). Frightened by her, the men abruptly leave the store and the camera cuts out, setting up the events for Paranormal Activity: The Marked Ones.

Question: What color is the symbol that Robbie draws on Wyatt's back?
Answer: Green