Actually the way I would do it is to, after pre-shading, outline and paint only those areas which will be grey. You may wish to lightly pencil mark the areas, outline with a light coat of paint and fill in the contours with a light coat. Once the gray is dry to handle, go back with the green and butt a light coat against the grey outlines and fill in the contours.
Remember the whole idea of pre-shading is to allow the darker shading paint to show through the more lightly applied top coats. If you paint all of the aircraft gray (or green) and go back and go back and paint the opposite color you have two coats of paint obscuring your shading attempt. Less is more.
Hard edges can be done with masks over the first applied colors. Softer edges can either be done with mask edges lifted by a snake of poster putty/blue tac or a low-pressure, closely held airbrush