Kenneth Johnson originally wanted the Hulk to be coloured green, not red. Bear in mind that this was before Thunderbolt Ross became the Red Hulk in the Marvel 'Earth-616' universe.
In a conversation with Stan Lee, Johnson famously said, "Stan, what is he the Envious Hulk? Why is green? Is he the jealous Hulk? ... The colour of rage is red, he [Banner} would be flushed with anger, and ... it's a human color, green is not."
Stan Lee responded by telling him the only reason the Hulk was green was that the original creature was grey, and the printers could never make a consistent grey colour so coloured the Hulk green instead. Johnson wasn't allowed to make the Hulk red, so he had to stick with the green Hulk.