Just a quick question for you. The color 'olive drab', the model master acrylic paint, looks like it is a grey-green color. The tamiya version looks like a dark green mixed with some brown. Which one best represents the actual 'olive drab' as used by modern U.S. Army / Marines.

I'd buy the Model Master. In my experience, they try and match the FS colours much more so that Tamiya.

Personally I use both, plus Poly S and some craft acryilics I've found. The one thing about Olive Drab is it came in so many shades, and faded differently each way, that just about any color will work.

The same debate took up about 6 months in the letters pages af Military Modelling, and and article by Steve Zaloga, to sort out. Tamiya is pretty close to 'factory fresh' paint, but the pigmants used in the original meant that the paint started to fade as soon as sunlight or rain hit it. Use the colour that you feel suits the model and conditions best.
Personally I have three jars of Tamiya, each with a different amount of desert yellow mixed in with it.