What I am missing is a good understanding of doing math on colours. Yes, if you break them down to tuples and then subtract, it makes sense. But connecting a pure math node (add / subtract / whatever) to an image leaves me in the dark still as to what exactly is going on.

What was being suggested was:

Surface.Bump = .0012 * Sub(ImageMap("MyColourPic.jpg"))

Sub in this case assumes Value_1 to be 1? and Value_2 to be 0? Which is the same as 1 - 0 = 1, which isn't doing anything?
Well, I plugged this all in as suggested and got the same image. Not trusting my eyes, I'm waiting for a better way to 1. validate that this way is erroneous and 2. create a greyscale image for those channels. Or do the channels just use what they need and ignore colour information anyway?

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