Right. So, one of the conclusions we can draw from that last experiment was that the Alternate_Diffuse channel on the PoserSurface node doesn't do much else than just accept whatever we put into it - in this case: a color_math Add node that adds pure red (1, 0, 0) and pure green (0, 1, 0) to give pure yellow (1, 1, 0). That said, the Diffuse_color channel must be doing something more. What that 'more' is we'll leave to Those-That-Know to enlighten us with.

I've learnt from BagginsBill that in order to let the Alternate_Diffuse channel to have complete control over what gets put onto the material's surface, you're best turning the Diffuse_Color channel off. How, you ask? There are a number of methods, actually - if you stick with us, you'll eventually find out how, and how and when to use one technique versus another - but the simplest is to simply set the Diffuse_Value channel value to 0, which turns that whole channel off. Now, the Alternate_diffuse channel has complete control.

Since my experiments in the PoserSurface node has left me more puzzled than anything about what's really going on under the surface, let's just assume - based on behaviour - that the Alternate_diffuse channel gives us complete control over what we're doing.

Agreed?

Now, we've got this so far:

We now know (or at least we think we know) what the behaviour of the color_math Add node is. That is: what you are seeing and what it is doing is: simply doing the maths I've outlined above. We are adding red and green to give us yellow. We've got a grip on one node.

What happens if we turn the node into a Subtract? What is 0,1,0 minus 1,0,0?

0,1,0?

Red: 0 - 1 = -1 ... does Poser allow negative values?

My experiments show no. It seems that subtracting that value (1) from 0 only leaves 0. Have a look at this image. I'm subtracting red from green. There is no red in green. So, subtracting red from green leaves green. Not some weird shade of green but just green.

This is cool to know. Let's take this one step further. First of all, we're going to get clear in our Dummies minds what "Green" is to Poser. It's 0, 1, 0. We know that? Right? This is an absolute. To Poser, when you say "Green", it thinks 0, 1, 0. We can use that.

That color_math Subtract node is going to come in real handy if we want to take all green out of an image. Let's test it. We're going to subtract green out of yellow. What does that give us? Heck, I don't even need to do a render!

The Color_math node. Does pretty much what it's going to say it does. And at this point, the Alternate_diffuse channel on the PoserSurface node lets us see these effects for ourselves. Why? Bill knows. I'm sure he's preparing a carefully-constructed explanation for us, and for that I'm already considerably in his debt. At this point I'm just elated to have predictable results based on his guidance.  I just know that what he's going to have for us is going to be gold.

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks