How do you paint or paste a greyscale image on a color channel?
Selecting or hiding channels in the channels docker has no effect.

Channels are currently... pretty much useless.

I think you could take a look at the different blending modes availeble, I think I saw a few for red green blue editing?

But at the very least till next release, don't expect much of channels , gimp is the better option right now.

Thank you, indeed blending modes and group layers provide a workaround.
For the record here the layers setup:

Doh, G channel group layer is set to Addition too

I noticed that if I create a generated layer with a pure red (#FF0000), green(#00FF00) or blue (#0000FF) color, and then check the layer properties or ctrl+click sample the color, it's slightly different than the chosen color. E.g. Create a pure blue #0000FF generated layer the colors becomes #1806FD no matter how many times I change it back to #0000FF.