For the olive drab-over-grey camo scheme on a subject like this old Monogram kit, I use Tamiya's rattlecan AS-6 USAAF Olive Drab, or their XF-62 Olive Drab, applied with the airbrush. For me, these are a starting point, a base color, over which I will apply weathering, including fading. So I'm not as worried as some might be, that the paint is an exact match right out of the can or jar.
For the underside, I use Tamiya's rattlecan AS-7 Neutral Gray, or XF-20 Medium Gray, again, applied with the airbrush.
Hope that helps!