Hmm... that's strange.

Is
isn't green when you save it as a GIF by itself?
Huh. Well, the GIF format can only save images using a very limited number of colors, so maybe that shade of green is the closest it can get to the color that part of the banner's supposed to be? And when you save it as animation, the resolution gets even more degraded?
That probably didn't really help you much, sorry. But it's all I can think of right now
