But look here! I subtract a different color than what I add. This gives me a new color of wood, but with the same type of color variations as the original.

Here I used a darker red as the basis. I also set the factor to 1.2. This gives me a nicely stained look.

You can't get this appearance with just multiplication or bias or gain - you have to actually produce negative colors as an intermediate step, using subtract.

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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)