It's not an attempt to go more realistic, it' an attempt to go more cinematic. Many gritty (and come that aren't) movies use Bleach Bypassing to create a desaturated, bleak feel. These movies are often color coded, such that the light is hinted with green, or blue, or orange, or whathaveyou (though for a number of reasons those are the three easiest colors to tint light with). Saving Private Ryan, for example, or Black Hawk Down, for another, contain incredibly desaturated colors. Most war movies do. Thus war games trying to go for that cinematic style (and don't let game developers tell you they're all bout realism. Bullshit, when they say that they're almost always going for realism with a cinematic boost) are "gray" with a certain color to them. MGS4 looks to have it too, according to the footage I've seen.
Atari 7800 (Sold), Intellivision (Thrown out), Gameboy (Lost), Super Nintendo (Stolen), Super Nintendo (2nd copy) (Thrown out by mother), Nintendo 64 (Still own), Super Nintendo (3rd copy) (Still own), Wii (Sold)
A more detailed history appears on my profile.