Why does this work for us? Let's see what happens to our color wheel image.

The green spoke (Hue = 2) gets multiplied with .25 resulting in a Hue of .5. The Hue at .5 (look on the original color wheel) is orange.

The yellow poke (Hue = 1) gets multiplied with .25 resulting in a Hue of .25. The Hue at .25 is brown.

So - greens become yellow-orange, and yellows become brown. Wow.

Higher Hues such as blue and magenta reduce to lower hues, resulting in green and yellow. But mostly we don't care because leaves are not blue or magenta.

As for the trunk of the tree - most bark is either brown or a shade of gray.

Notice along the bottom that the shades of gray are unmodified by this operation!!! That is because they have no Hue. The Hue is "nothing". "Nothing" times .25 is still "nothing". Hah! I love math.

Notice also that browns (low hue close to 0) become more brown (low hue closer to 0).

This is another amazing coincidence. When we do this hue multiplication, the bark of the tree is relatively unaffected!!!

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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)