What was used on the Incredible skin to make him green?

The Incredible Hulk's green skin is a result of gamma radiation. In the comics, Bruce Banner was exposed to a massive dose of gamma radiation from a nuclear bomb test, which triggered the transformation into the Hulk. The radiation alters his cellular structure and is what causes the green color.

It's worth noting that the exact mechanism behind the color change varies slightly across different comic book iterations and adaptations, but gamma radiation is the core reason for the Hulk's distinctive green skin.