The most common name for the shade of green that matches Master Chief is "Oregano" or "Olive". However, some brands of spray paint, like Rust-o-leam, don't have names for each color; you have to go by the color of the cap.
The best way to pinpoint your best color, would be to look for a shade of green (smooth satin finish) with subtle undertones of grey or even a little brown. Of course, you can always buy a few different colors, try them out on a test piece, then return the ones that don't match.
Good Luck!