I am looking for some advice please.
For the first time I have tried pre-shading my latest model, the Airfix 1:24 Hellcat ( http://cs.finescale.com/fsm/modeling_subjects/f/2/t/184475.aspx )
I have done the underside and am now moving on to the top side.
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Would you paint the whole top green, then do the grey. Or would you go grey first, then the green?
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I assume having done the first colour then by the time I put down the second colout then the pre-shading would not be visible? Should I then re-preshade the areas for the second colour before putting down the second colour??
Hope that makes sense 
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