The Incredible Hulk has become iconic for being the monstrous green rage beast Dr. Bruce Banner turns into whenever he gets angry and/or stressed. Thankfully over time, Banner has learned how to more or less guide the Hulk into a force for good, with the Hulk even becoming a frequent member of the Avengers. However, the Hulk wasn't always intended to have green skin.
According to reports from Marvel Comics' history, Marvel legend Stan Lee originally wanted the Hulk to be grey. Read on to learn more about why he became green, both the real reason and the narrative explantation that was made.
Back when the original Incredible Hulk series was created by Stan Lee and Jack Kirby in 1962, Stan Lee wanted Bruce Banner to turn into a monster akin to Quasimodo or Frankenstein. A monster that was misunderstood. This was also right off the success and popularity Fantastic Four's The Thing, and Marvel wanted to capitalize. In order to make sure the Incredible Hulk had a mass appeal, Lee wanted the Hulk to have grey skin, so as to not have him be identified with any one ethnicity. Unfortunately, the colorist had trouble with the ink during the first few issues, resulting in the grey coloring appearing as green. However, Lee ended up liking the green and it stuck.
Narratively, the explanation for the shift to green was that it was the result of continued gamma exposure on the part of Dr. Bruce Banner, and that was that. However, over time is was revealed that Grey Hulk was another gamma-radiated personality that would make its presence known over time, taking over the classic Green Hulk persona. While Green Hulk was angry, savage and more mindless initially, Grey Hulk was able to retain some more of Banner's intelligence. He even took on the name Joe Fixit for a time, becoming a gangster enforcer in Las Vegas. The Grey Hulk transformation was also triggered by Banner's willpower as opposed to Green Hulk's with his anger.
At one point, Banner's friend Doc Sampson tried to help him and his two personalities with therapy, especially when they were all fighting for control quite aggressively. Thanks to the therapy, all of the personas coalesced into the Professor, where Banner's mind was in control of the classic Hulk's body and strength. This same concept can be seen in the MCU's Avengers: Endgame, where Mark Ruffalo's Bruce Banner spent 15 months in a gamma lab and he found a way to combine himself and the Hulk into one being, much like The Professor version seen in the comics.
It's pretty fun to know that a simple mistake and problem with the colorist and the ink during the Incredible Hulk's origin would create such an iconic element to the Hulk's overall aesthetic and look. Not only that, but it created the inspiration for a whole new side and persona to Bruce Banner that probably wouldn't have come about otherwise.