The Incredible Hulk’s green skin color was originally the result of a printing error!
In the debut of Stan Lee’s The Incredible Hulk series, the Hulk fashioned a gray skin color because Lee did not want him to be in any way associated with a particular ethnic group. However, colorist Stan Goldberg had problems with the gray coloring, which resulted in a different colored Hulk in each copy.
Most of these colors were various shades of gray (insert obvious joke here). However, in some copies, the Hulk came out green. Upon seeing those copies, Lee became convinced that green suited the Hulk much more than gray. In subsequent issues (and television shows, films, posters, etc.) the Hulk’s skin color was the green we all know and might or might not love.