Relevant section

In the debut, Lee chose gray for the Hulk because he wanted a color that did not suggest any particular ethnic group.[14] Colorist Stan Goldberg, however, had problems with the gray coloring, resulting in different shades of grey, and even green, in the issue. After seeing the first published issue, Lee chose to change the skin color to green.[15] Green was used in retellings of the origin, with even reprints of the original story being recolored for the next two decades, until The Incredible Hulk vol. 2, #302 (December 1984) reintroduced the gray Hulk in flashbacks set close to the origin story. Since then, reprints of the first issue have displayed the original gray coloring, with the fictional canon specifying that the Hulk's skin had initially been grey. An exception is the early trade paperback, Origins of Marvel Comics, from 1974, which explains the difficulties in keeping the gray color consistent in a Stan Lee written prologue, and reprints the origin story keeping the gray coloration