Nice job. Of course, desaturating the green a bit would make it look more like the default color of the Chief's armor, not the multiplayer green.
Also, as Pezza said, the red hue of the visor makes it appear as if you altered the contrast of the image. The red doesn't really fit, given the color of the light reflecting off the visor itself. With the bright light, the color should be a bit washed out(desaturation).
These are just suggestions for furture alterations.
Overall, nice job.
[Edited on 5/14/2006]