@hardywang First of all thanks for starting this topic! RGB Channels is one of my favourite tools in RT (mostly because it enables me to do some Jose Villa shooting Fujifilm 400H film magic to images by making greens bluer and by making skin tones goldener by taking blue out of red)
The image is not mine btw 
it’s from one of Jose Villa’s Workshops as per link above.
Anyway…
I find this thread useful, because it helps me wrap my mind around what RGB Channels actually does. Most articles online seemed helpful but not altogether fully enlightening, so I mostly play it by ear and make things work to look okay without knowing 100% for sure what I’m doing (shh don’t tell my clients).
The way I see it: (Please forgive my ignorance good people, where ignorance may be, and yes, critique and corrections are welcome)
RGB Channels is a tool that lets you define how much of each initial colour is placed in each channel. As an example, the RED channel would be as such:
Red Channel: (what I called initial colour -or all the red that’s available)
Red: 100% – (here I distribute 100% of it)
Green: 0% – (here I distribute 0% of it)
Blue: 0% – (here I distribute 0% of it)
This would mean, since we talk about the red channel, we discuss how much red we put in each channel. So according to the above, we put all of the red channel (all the red colour there is to be worked with) into the final red output, while none of the red is added to the final blue output or green.
It took me a while to figure this out, so that’s why I am over-stressing it. There are two reds; one is the altogether redness that is in the beginning, and then there is the final red amount for pixels. This sounds confusing even to me, so let me use a different example; My turning the greens bluer for filmy effect. Here’s what I do:
Red Channel:
R: 100
G: 0
B: 0
Green Channel:
R: 0
G: 100
B: 0
Blue Channel:
R: 0
G: +40
B: 100
So the way I think of it when I do this is the following; I go to the channel that I wanna do stuff with, in this case it being blue; and then I do stuff with it, in this case add some into green, in this case adding +40 in the green section of Blue Channel.
Considering the previous comment by @Morgan_Hardwood about how making changes to colour channels, I guess I would have to cancel out the channel imbalance by also making the B: 60 (because 100-40=60).
This seemed a bit counter intuitive to me at first: I used to think; "If I wanna add blue to the green tones, well I’ll go to the Green Channel and make it had more blue in it, such as
Green Channel:
R: 0
G: 100
B: +40
but this turned out would add green channel to blue output.
So if I got this somewhat right, the rule of thumb is this:
When dealing with RGB Colour Channels in RawTherapee; when you look at one of them, say Red Channel, you think that this is the colour you are about to work with; and when you edit the R, G, B values within it, you think of those as where you add said channel to.
P.S.:
I’d still love to have sliders that allow you to modify the hue of basic Red, Green, and Blue to other hues, Photoshop/Lightroom style But that’s another topic…