This month saw the return of Mark Ruffalo’s temperamental Hulk in Avengers: Age of Ultron. His character has been a fan favorite for over 50 years—but did you know that, originally, the big guy was given a very different look?

The muscle-bound antihero made his grand debut in Marvel Comics’ The Incredible Hulk #1 (1962). “Is he man or monster or both?” asked the cover, perfectly capturing the inner turmoil of this future superstar. Appropriately, creator Stan Lee says that two classic creature features inspired his moodiest brainchild.

“I’ve always had a soft spot in my heart for the Frankenstein monster. No one could ever convince me he was the bad guy,” the writer said in his book, The Origins of Marvel Comics. “He never wanted to hurt anyone; he merely groped his tortuous way through a second life trying to defend himself, trying to come to terms with those who sought to destroy him.” Lee imagined a cross between that sympathetic beast and Robert Louis Stevenson’s shape-shifting Dr. Jekyll and Mr. Hyde. The result was a comic book icon like nothing anybody had ever seen before.

But one key ingredient remained missing. Previously, Lee hadn’t bothered dressing up his Fantastic Four characters in anything more elaborate than normal street clothes for their first appearance. Big mistake. “The fans said, ‘We love the book. It’s great. Oh, it’s the best new thing we’ve seen. But if you don’t give them costumes, we’ll never buy another issue,’” Lee said. An angry, marauding giant like the Hulk would have little use for tights and capes. So, Lee chose to appease readers by instead giving his Hulk a strange skin color. Gray—being “spooky”—was his top choice.

But when the time came to release the first Hulk issue, the shade of gray Lee wanted wouldn’t print correctly. On certain pages, the Hulk rocked a light, silvery complexion. On others, he was charcoal black. Clearly, this wouldn’t do. After reviewing Marvel’s extant superhero lineup, Lee realized that green wasn’t being used by any major characters. Boom—problem solved!

Lee asked colorist Stan Goldberg to make the Hulk green for issue #2. “It turned out to be a great choice,” Lee said. “By making him green, I was able to give him nicknames like ‘The Jolly Green Giant’ [or] ‘The Green Goliath.’ So, I’m very glad we made him green.”

Still, fans hadn’t seen the last of gray. In 1986, alter ego Bruce Banner unexpectedly turned into a clever, granite-colored hulk who called himself “Joe Fixit” and started working as a Las Vegas bouncer. Hopefully, this won’t come up in Avengers III