Hulk is one of the founding members of the Avengers and one of the most recognizable Marvel Comics characters of all-time, but many fans of his appearances in live-action films and television may not realize that he wasn't always green.
The alter ego of Dr. Bruce Banner was originally intended to be grey but due to issues with inking comic pages in the early '60s, his color was changed by creators Stan Lee and Jack Kirby to be green. The grey color however, remained part of the character's identity and history, and as we learned over the weekend, almost made its way into Avengers: Age of Ultron as well.
There have been many Marvel Comics storylines over the decades that delve into the mindset of Banner, some of which even point towards the character having dissociative identity disorder long before a science experiment granted him the ability of turning into The Incredible Hulk. So, his personality issues are not as simple as Banner vs. Angry Hulk.
The Hulk himself has different personalities, and each can considered a different aspect of Banner's own psyche: the angry and green (most common) savage Hulk we see in the movies, and another, more calm and intelligent Grey Hulk persona who at one point wore a suit and served as a bouncer named "Joe Fixit" in the comics.
Speaking with the Industrial Light & Magic special effects crew who worked on the Avengers films, Wired learned that during at least one sequence in Age of Ultron, the Hulk was almost going to change color to grey when he hit a whole new level of rage.
"Initially, Hulk was actually planned to turn grey when he's angry Hulk in the Hulkbuster sequence but hey, everybody wanted the green guy. So, in the end, ILM just changed his eyes to make them look more sullen when he's under the control of Scarlet Witch."
Without getting into spoilery details, the Hulkbuster sequence - as revealed in Avengers: Age of Ultron trailers and clips - features the long-awaited bout between Iron Man suited up in his largest armored suit yet battling the Hulk in an effort to prevent the uncontrollable beast from hurting civilians. For more on this, see: Hulk vs. Iron Man Hulkbuster Fight Explained.
Seeing another color of Hulk wouldn't be a surprise to us for the Marvel Cinematic Universe given the company's desire for each franchise film to introduced new looks/designs/costumes for almost every character for marketing and merchandising opportunities. After all, there are quite a few versions of the Hulk in the Marvel Comics.
Since Mark Ruffalo has several pictures left on his contract with Marvel, there's still room for his Hulk to evolve in design. Whether that means a new color of skin, weapons (see: Planet Hulk/World War Hulk comics) or even an advanced armor suit (see: modern Avengers comics), there are plenty of ways for the MCU to augment the strongest Avenger.
Marvel’s “Avengers: Age of Ultron” stars Robert Downey Jr., who returns as Iron Man, along with Chris Evans as Captain America, Chris Hemsworth as Thor and Mark Ruffalo as The Hulk. Together with Scarlett Johansson as Black Widow and Jeremy Renner as Hawkeye, and with the additional support of Samuel L. Jackson as Nick Fury, Don Cheadle as James Rhodes/War Machine, Cobie Smulders as Agent Maria Hill and Stellan Skarsgård as Erik Selvig, the team must reassemble to defeat James Spader as Ultron, a terrifying technological villain hell-bent on human extinction. Along the way, they confront two mysterious and powerful newcomers, Wanda Maximoff, played by Elizabeth Olsen, and Pietro Maximoff, played by Aaron Taylor-Johnson, and meet an old friend in a new form when Paul Bettany becomes Vision.
The Avengers: Age of Ultron releases in theaters on May 1 2015, followed by Ant-Man on July 17 2015, Captain America: Civil War on May 6 2016, Doctor Strange on November 4 2016, Guardians of the Galaxy 2 on May 5 2017, Spider-Man on July 28, 2017, Thor: Ragnarok on November 3 2017, Avengers: Infinity War - Part 1 on May 4 2018, Black Panther on July 6 2018, Captain Marvel on November 2 2018, Avengers: Infinity War - Part 2 on May 3 2019 and Inhumans on July 12, 2019.
Grey Hulk art by John Gallagher.