Well , not in general, not all color-based channels.

This isn't about matmatic. This is how the node works.

Spots produces one of two colors. If you pre-multiply both, that's the same as post-multiplying the spots with something.

You can't generalize this, however. For example, pre-multiplying each input to a Velvet node will not produce the same as post-multiplying. The reason is the internal math of the velvet node is not simple addition. But the internal math of Spots is.

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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)