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Well, if you've already flattened your layers, then you'll definitely have to start over. But you know the process for getting to this point, so that shouldn't be too much of a problem. One thing you can add, along the way, is the Render > Difference Clouds filter, which does what you want - switches the colors. But, it makes the image look different each time you run it, so you'd have to make new blended layers, anyway, to get your slimy look.
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