He would change color in the comics, sometimes it depending on the artist, and sometimes he would be grey (although not often willingly), maybe it has to do with the source or intensity of his anger. OR like Brain said they just felt more of an olive hue would be more realistic. Of course there is always the marketing factor, no character has looked the same in any two movies yet, why should hulk be any different? (the only exception so far would be Mark VI between IM2 and Avengers, but VII was new)
I know it's a long shot, but i'm still hoping he does go savage grey in the movie. I could care less that his grey color in the comics made him Mr. Fixit. I never really cared for that version of him anyway.