Actually, the Hulk was green by his second issue. The change was made because it looked better and because the coloring technology of the time could achieve a more consistent green than grey.
While it’s correct to say the Hulk was more or less intelligent during various periods in the 60’s, the idea of identifying various periods or linking them to his other characteristics is something that’s only done in hindsight and doesn’t really fit the text. At the time, the Hulk’s intelligence and demeanor varied wildly from issue to issue – not via any plan, but because of Silver Age Marvel’s strange and chaotic production process. (Stan Lee would hand his artist a half an idea, the artist would draw something, and then Lee would spend 20 minutes writing dialogue which fit the story.) This process led to a pretty inconsistent set of comics. The Hulk suffered in particular from this problem, both because he always was somewhat undefined, and also because he had rotating artists (Kirby, Ditko, John Buscema, Romita, Kane, Marie Severin, etc. all had their shots illustrating the character). Sometimes the Hulk was childlike, merely wanting to be left alone. Sometimes he was aggressive and violent. Sometimes he spoke in complete sentences, sometimes in monosyllables. While he never had the intelligence of his nuclear-physicist alter-ego, in some issues he was no dumber than your average street thug. The Hulk’s transformations into Banner and vice versa were also inconsistent – sometimes they were triggered by nightfall, sometimes by additional doses of gamma radiation, sometimes by becoming angered. Indeed, in some early issues, when the Hulk got angry it would make him revert into Banner!
It was in the 80’s, when Peter David tried to make sense of all this early variability and shoehorn in an explanation for it that the idea of different Hulk personalities emerged. David’s version of the grey Hulk, who went by the name “Joe Fixit,” had average human intelligence and super strength (though nowhere near the green Hulk’s power), lots of cunning, a vicious streak, and IIRC he appeared regularly at sundown. He got a job working as a strongarm man in Vegas.
In Paul Jenkins’ recent underrated run on the title, he brought back the idea of the Hulk(s) being a sympton of Banner’s multiple personality disorder; Banner, Fixit, the childlike green Hulk, the “merged” Hulk, and a number of other personalities. In the current run by writer Bruce Jones, all of this complexity has been shelved for the time being; the Hulk is nothing but a savage destructive force of nature, and most of the time the book focuses on Banner and his efforts to keep himself calm enough that the Hulk can’t get out.
–Cliffy