UPDATE: July 26th
OK, more stuff done on the back of the helmet. I had transferred the pencil lines onto the back panels, and today I painted on some Winsor & Newton's masking fluid. I think this stuff is very easy to work with. It flows easily and it's easy to control the application with a small brush.
This pic shows the areas that are masked. It shows as darker and kinda glossy.
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Once the masking fluid was dry, I sprayed on my custom bluish green color. I tried to match this tone as closely as I could to pics of the screen used helmet. I started with a base of Testors Euro I Dk Green, and added some Floquil BAR Blue a few drops at a time until I thought it looked correct. Then my secret blend of 11 herbs and spices, and we're cooking!! 
These pics show how different the color can look in different lighting.
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After several hours, I took the tape and plastic wrap off, and rubbed off the masking fluid. I've never had any problems with the fluid sticking to my undercoat or anything. To remove the masking fluid, I just use a little scrap of tracing paper and rub it in small circles over the surface. It gives just enough texture to "grab" the fluid without scratching the surface. It basically just rolls off in little dried blobs.
And here's what it looks like now.
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I'll go back and pore over my reference pics to see if there are any touch ups needed. Then the next step will be to put on the Panzerolivgrun on the dome and lower cheeks! 
