Maria Hill mentions that the Hulk was originally grey-skinned. In the comics, the Hulk was in fact grey-skinned at first. However, his skin was eventually changed to the color green to difficulties publishers experienced in printing the color grey in comics during the 1960's.
Bruce: How do you get to say no to me? I'm an Avenger, I was an Avenger before you.I want to help.
Hawkeye: Ok A you're not an Avenger, the Hulk is. Calling you an Avenger is like saying my bow's a member of the team. And two, Cap's right, you got to stay here.
Bruce: You're making it very difficult not to turn into the Hulk, and tear you apart.
Red Hulk: You've lost! Do you hear me? You're nothing!
Hulk: You're wrong. I'm an Avenger!
Hawkeye: Is there really another Hulk running around?
Captain America: A better question is, how do we track it?
Wasp: The Avengers ID cards. Banner's got one. We can trace the signal. Man, if I'm the one coming up with this stuff, we're really in trouble!
Wasp: (to Red Hulk) All right big guy, who are you, and why are you doing a horrible job impersonating the Hulk? I mean, for real, even blue would have been a closer match!