A grey Hulk was originally included in Joss Whedon's "Avengers: Age of Ultron."
According to Screen Rant, the shade of grey was initially indicative of Scarlet Witch's psychic powers on the Green Giant during the incidents in Wakanda, which resulted to his clash with Tony Stark's Hulkbuster suit. However, the concept would be puzzling for the readers of the original comics, a reason why it was scrapped by Whedon and Marvel Studios.
In the comic book "The Incredible Hulk," the grey version of the colossal Avenger stands as Bruce Banner's another split guise, which was capable of controlling his thoughts, in contrary to his green form. As per Cinema Blend, this fact on Hulk's grey sort would not be applicable to Whedon's "Avengers: Age of Ultron," since Hulk was under Scarlet Witch's mental influence and was incapable of controlling his mind.
At this point, it is still unknown as to how The Hulk would come back to Marvel's Phase Three films, since he left Earth and went to somewhere outside the planet. Previous reports said that the green Avenger has reached the cosmos where the Guardians of the Galaxy are; however, such reports have been debunked.
With Hulk's whereabouts still unknown, his fellow Avengers are set to be seen in Joe Russo and Anthony Russo's upcoming Marvel Comics film "Captain America: Civil War." which follows the release of the Accords.
The Accords refers to the new law set by the government to bind the superheroes in using their superpowers and abilities. In addition, it serves as the film adaptation of the Superhero Registration Act that was released by the government in the comic book.
Directed by the Russos, "Captain America: Civil War" is slated to premiere in theaters in the United States on May 6, 2016.