If you take the textures from the plain white colour scheme you can use it as an overlay and paint your own stuff underneath it.
It'll depend what program you're using but once you've opened the files, add a layer beneath the base layer (you may need to convert the base to a layer in some programs) then change the blend option for it to multiply (generally right click on the layer and then layer properties or somesuch, then they'll be a drop down for blending options).
Basically multiply treats white as transparent with greys having a varying degree of transparency until you get to black which isn't (technically it's a bit more complicated but that's the basic effect you see). This means you can paint your stuff beneath it and the rivets and panel lines etc. will still show up properly.
I know this works in photoshop and paint.net, I'm guessing paint shop pro and gimp have similar options.
