See here:
Maususers Battle of Britain Mods
It is a PSD file containing the layers. I tried it with GIMP and it works

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Multiskin means you can paint specific aircraft (say the one of Galland), specific units, specific times (say the fairly short time when that one unit disbanded use of the Hakencross to make a statement) etc. Of course we already have quite a lot, so if you want to make a skin that "fits in", have a look at the available and WIP textures or ask in this thread.