Ok. This simple RGB model is the best tool to understand what needs to be changed:
And as you can see, yellow is the combination of of equal values of red and green in the absence of blue.
Orange color you get - similar to how you mix colors in painting - by mixing in a bit more red. So, orange is more reddish yellow, so to speak. That is, if you want to get orange from yellow, you have to increase the value of red.
The problem is - if I want to do that in the example above, it doesn’t work, because we have very bright yellow, which means both the red and the green is already at maximum. So, I can’t increase the red:
But, what I can do is decrease the amount of green and thus the amount of red becomes higher than green and I get orange:
So now I got orange from yellow but as you can see, green became darker and also other colors where the green was involved have changed:
- white (where all 3 channels are mixed together with equal share) got magenta
- cyan (combination of blue and green) got blue
The same result as above with curves can be obtained with channel mixer. I can use input green in green channel (here in GIMP is called Green in Green channel) to reduce proportion of green:
Thus I changed - just like in the example above - all colors where the green channel plays a role.
But if I use input red in the green channel (in GIMP Red in Green channel) instead of input green, I only affect the areas where the red channel was involved:
So, yellow becomes orange and white becomes magenta. the cyan (combination of blue and green) is not affected.
Now to answer your question where to start. Here is a concrete example. We have an autumn scene and the leaves are too orange. We want to make them a little more yellowish.
So, we need to increase the amount of green or decrease the amount of red to get yellow. Now the question is, what other colors do we want to influence as well?
In the green channel we can use input green. This will affect all the areas where the green channel is involved:
That is, next to the yellow leaves, the sky also became greenish because the green channel is also present in the sky (combination between green and blue).
But if we do not want to influence the sky, we can use input red in green channel:
Since we are affecting the areas where the red channel is present, the sky will not be affected, but everything that was red before will become orange because this is the area where logically the red channel has the highest value. Correspondingly, - if we use these proportions of the red channel for the increase of green - the increase of green value will be strongest there:
Another possibility would be to reduce the red in the red channel.
If you want to protect the sky, you can use input red, because red channel is not present in the sky. However, this will make the photo a little darker:
