Okay, let’s simplify things even more:
Besides the very nice from @st.raw here is one more:
I’m using GIMP channel mixer here because it’s easier to keep track. As you can see, as a starting point there are 3 active channels, red in red, green in green and blue in blue. On the picture you can see the composition of the colors. Sky is blue, grass is green and yellow:
Now watch what happens when I switch off the green channel (only red and blue remain - green in green =0).
Now we only have blue, red and combination between these two (magenta):
And now I add the green channel to the blue one (green in blue =1) And as you can see, the green grass is now blue too. So, we still have only two color channels (red and blue) but with the difference that I added the green channel to the blue and the green part of grass is now part of blue channel!:
So, in this simple example, I mixed two color channels together to achieve a certain effect.
Here is another example with red and green. We now have in the picture only red, green and the combination of both (yellow):
If I want the sky to be more “reddish”, I add blue to the red channel, because the sky is blue (for the most part) and the sky is now part of the red channel:
Now these were just two simple examples with only two channels to understand the logic. You can combine all three channels in countless ways by mixing three colours with each other. There is no mystery behind it, but, you have to take your time and experiment to find out how to combine the colors well. In painting, too, you have to learn this first.