The Hulk Was Originally A Gray Werewolf
Let's face it, the Hulk isn't exactly a complicated character to make. Take a little smart guy, occasionally turn him into a big dumb guy, and paint him green so that the ghost of Robert Louis Stevenson doesn't sue your ass off.
But even with the basic idea of the Hulk being a copy of a copy so faded that you can't read the copyright anymore, he didn't come out of the womb fully Hulked out. When he first lumbered onto the comic book scene, the Hulk was a drab, battleship-esque gray, and didn't become the mascot for a fluorescent paint factory until the second issue.
Why was the Hulk first going to like an angry block of cement? There's a variety of explanations. If you ask Stan Lee, it's because he thought the gray would look better than it did, so he switched the color out for the much more appealing snot green of later issues. If you ask others, it's because Marvel couldn't produce a consistent gray tone issue to issue, due to either their shitty printers or shitty paper quality. Of all the colors to not get right, it's pretty ironic that the Silver Age of comics couldn't nail light gray.
And it wasn't just the color of his body that the creators couldn't keep consistent; it was the same with the nature of how the Hulk transformed. They eventually settled on "the rage of a dad when he sees someone has messed with the thermostat" as the Hulk's trigger. In his earliest iteration, however, he transformed whenever the sun went down, like he was some sort of weird lycanthrope with alopecia and a green rash.
Marvel ComicsHe loses an hour during Daylight Smashing Time.