@tstoddard - don’t get too caught up in the idea of color models/spaces just yet. It can be counter-intuitive to wrap your head around but at the end of the day the biggest things to keep in mind is
adjusting each channel in the mixer (red, green, or blue) will only adjust how “bright” that particular color is.
You won’t be adding any other colors into the mix. The channel mixer only let’s you adjust how intense that particular color is.
Hopefully I can clear this up. Let’s consider four “pixels” that are only made up of a Red and Green channel (we’re ignoring Blue for the moment to simplify things) that go from 0 – 255.
From left to right, we are going to set their initial Red, Green values as:
First: 127, 0
Second: 127, 64
Third: 127, 127
Fourth: 0, 127
So, let’s look at modifying only the Red channel in the channel mixer. I’m going to use GIMP for this example but the theory should be the same everywhere.
So on the Red channel, let’s push the Green contribution up to “50”.
What I would expect from this is to take the Green channel value for each pixel, multiply by 50%, and then add it to the Red channel.
First Pixel
So let’s look at the First “pixel”.
It’s values are R = 127 and G = 0. So…
- Multiply Green channel value by 50%:
0 × 0.50 = 0 - Add the result to the Red channel value:
127 + 0 = 127
The final result of this operation on the pixel will be nothing:
R, G = 127, 0
Second Pixel
The second pixel has something interesting finally happening to it.
Remeber, it’s values are R = 127 and G = 64.
- Multiply Green channel value by 50%:
64 × 0.50 = 32 - Add the result to the Red channel:
127 + 32 = 159
The final result of this operation on the Second pixel will be:
R, G = 159, 64
Third Pixel
The third pixel has it’s values as R = 127 and G = 127.
- Multiply Green channel value by 50%:
127 × 0.50 = 64 - Add the result to the Red channel:
127 + 64 = 191
The final result on the Third pixel is:
R, G = 191, 127
Fourth Pixel
The fourth pixel has no Red in it: R = 0 and G = 127.
- Multiply the Green channel value by 50%:
127 × 0.50 = 64 - Add the result to the Red channel:
0 + 64 = 64
The final result on the Fourth pixel is:
R, G = 64, 127
If we actually do this in GIMP, we see that this is exactly what we get (notice that the Green channel hasn’t changed at all in this only the Red channel).
Top, after channel mixer +50% R(G), bottom original.
If we go further and instead adjust the Red channels Green contribution, R(G), up to 100%:
First (R, G): 127, 0 ⟹ ( 127 + (0 × 1.00) ), 0 ⟹ 127, 0
Second: 127, 64 ⟹ ( 127 + (64 × 1.00) ), 64 ⟹ 191, 64
Third: 127, 127 ⟹ ( 127 + (127 × 1.00) ), 127 ⟹ 254, 127
Fourth: 0, 127 ⟹ ( 0 + (127 × 1.00) ), 127 ⟹ 127, 127
Which is what we see when doing the operation in GIMP:
Again, in all cases, notice that the Green value doesn’t change - we are only modifying the values of the Red channel.
Hopefully this makes sense?