Hey Martin ...
You did nearly what I wanted .... I really need to learn to use graphics programs with more proficiency 
But the bottom part of the folder, you have kept as the back layer, so if I make the Back Layer Red and the Front Layer white, I get a Red line along the bottom.
Here is what I mean (rather crudely made with a pencil
):

The "Black" part is my Front Layer, The Red is the Back Layer and then the blue is the "Line" Layer.
So I would put them as follows (bottom first):
1. Front Layer, so any image I would put on at the top edges would be "cut" as the parts exceeding it would be covered by the Back Layer
2. Back Layer, so I would may get images that look like the Firefox one from my previous post, where it is "cut off".
3. The grey line layer, so it would cut over any images I put on the Front Layer.
First of all this would hopefully allow me to make folders in the same style as the orginal creator, with "cut-off" images and difference in colour scheme between the front and back layers.
Edit: I think I figured it out myself
.... I cut out the top by using the green/white Sony folder so it was easy to see where the white and green split ... then erased the grey bordering the green parts, and then used the magic wand in paint.net to cut the green part out and save it seperately.
Now I have the normal borderless generic folder, and I can then just copy the green top onto that in a new layer, so they are independent. Did the same with the grey line. Thus I can create an icon like I explain above, with graphics and overlays that only affect part of the icon ... or just move the overlay layer on top if I want it to affect the whole icon
....