There's probably a thread on every forum, here's one: http://www.ar15.com/forums/topic.html?b=3&f=118&t=515847
Why I Chose Foliage Green, by Tirod
I choose Foliage Green because real professionals don't use Black, it's real noticeable against the newer lighter colored camo's especially. OD is OK but comes off dark also, and the human eye is set up to see a dark objects against light backgrounds.
OD worked in a woodland environment with lots of evergreens, such as Europe. With use in semi arid countries, a lighter color is mandatory, so much the Army finally formalized what experienced troops and special units did - paint the weapon to blend in. In civilian use, three or four season use means not having dark greens as a dominant color. Fall foliage or winter is predominantly browns, tans, and greys. To meet the extremes of both winter and summer, Foliage Green does both the green of summer and the grey of winter. In contrast, FDE/tan appears as a unnatural earth tone - disturbed earth or bare ground.
IMHO.
Possibly the biggest effort in choosing a color is getting the upper and lower to match. They can be anodized, but most receivers are cheaper in mainstream black. Krylon doesn't offer a good match, either. My solution was to color match using the stock as a source. I work at an auto parts store, and had the guys in the Paint department make up a 1/2 pint of fast dry to match - which it does. Degrease and apply directly to the anodizing, which is the industry standard primer for industrial/commercial overcoats on aluminum. Do Not Sand. It dries in less than 15 minutes, apply a coat or two of Matte Clear Krylon over the top. It flats the color and adds a great deal of scuff resistance to the finish.
I can't recommend using a foam brush to apply, something in a 1/2 smooth bristle would work much better. Unless you like the patina of field applied paint.