Joss Whedon is one of us. He's a geek through-and-through.
Story is king with this guy, make no mistake. But when he's crafting his Avengers movies or S.H.I.E.L.D. TV show or X-Men comics, he's always on the lookout for ways to give the true believers a little something.
In Avengers: Age of Ultron, Whedon originally wanted to pay respects to the Hulk's comic book beginnings by having him go gray for one sequence. As comics readers know, when the Hulk originally arrived on the scene there was no green in sight. The green goliath started out his career as a big gray monster.
How do you have the Hulk go gray when he's so well established as green-skinned? Whedon's idea was to incorporated it into the Hulkbuster sequence. (Minor spoilers ahead.) In the film, Scarlet Witch — while still fighting against the Avengers, instead of with them — uses her powers to get inside the Avengers' heads and show them nightmarish scenarios that really mess them up.
No one suffers worse effects from this, perhaps, than Bruce Banner. What she shows him sends him into an uncontrollable Hulk rampage, causing Iron Man to break out the Hulkbuster armor to stop him. Whedon's thinking was that maybe Hulk's skin color could change when he's sent into a blind, Witch-induced rage. It would be a visual cue that this isn't the friendlier, more heroic Hulk the Avengers have come to accept — while giving those loyal comic fans a few good ya-yas.
The revelation comes from a behind-the-scenes video posted by Wired, which features ILM's visual effects magicians explaining how the giant CGI hero was created. The video also reveals that gray Hulk's inclusion was nixed when the filmmakers realized just how popular "the green guy" is. So instead of changing his skin, they applied some subtle changes to his eyes.
What even the hardest of the hardcore may not remember is that the difference between gray Hulk and green Hulk in the comics was explained as a sort of alternate personality thing, where different colors would represent different personas. Gray Hulk is a smarter, more cunning monster, while green Hulk is the rage monster.