It's the polarizing film used with green backlights. Same thing with Blue and probably some other ones as well.
The polarized film is generic and not color specific.
rlock999,
Is the polarized film in the 'inverted' position? When your DMG is off, the screen will be a dark blue color.
The downside to the backlight modification is the mirror foil is removed to allow light to shine through the LCD. The mirror foil is what makes the screen more visible in daylight. The backlight mod is geared towards people who use their gameboy at night and play gigs. If the polarized film is in the normal position, it's much more visible in daylight. If it's in the inverted position, it's more difficult to see.