Here are a bunch of variations using this same color map and shader.

The bottom row, the only thing changing is the factor. The subtract and add color are the same.

In the upper row, the add color is different from the subtract color.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)