So, RobynsVeil, it's not the intermediate stuff you're talking about. Correct?

I've spent an eyestraining half-hour colour-picking colours from one preview and another, and the change from the source is not making much sense except if it's as bagginsbill says and it's a preview issue. It's difficult to see what exactly is taking place because the source texture is (I would guess) much larger than the shape it's been previewed on so it's not being displayed 1:1, making a direct pixel-to-pixel comparison impossible. Some solid colours would be better to see what changes are going on in your brick network between source and output.

 

Further: If I sample (not very accurately, I must admit!) the colours that occur in one area of the preview and then sample (again, very approximately) the colours from the same area on the other preview, it appears that something is changing in the red and blue components, too. Which is strange if all you're doing is subtract 128 of green. You'd expect the clamping to show up (if that's what Shader Mixer does) in the green component and leave the red and blue alone. But that's not happening. Every component of the source image is changing.