olive drab/ neutral grey US ww2 aircraft

Try ModelMaster enamels.  They have both OD as well as a faded OD.  To my eye, their colors look reasonably close to what I've seen from decent WWII color photos.

If by "looks right" you mean a finish that appears maybe a bit sun faded and not quite new without looking as though the plane has been sitting in the desert for 50 years, you can if your careful do some cool effects with light oversprays of slightly lighter colors.  So after you've painted the base coat, you can mix a little white or maybe tan with the OD, and give a light coat basically trying to give that ever so non-uniform appearance that real aircraft would have.  You make the effect subtle as you like.