Please help me understand how to use a color channel as a mask

[Mapsking]

Hello, I am a novice GIMP user.  I am modifying game textures, which usually include a mask texture, with various red, green, or blue colors.  Those are used by the game engine to color specific parts of a texture to customize it.  I was manually editing parts by hand, which is not accurate, and someone showed me the short video in PS below.  I don't really understand it, but maybe someone can watch it, and can show me how to achieve the same thing in GIMP with step by step instructions?  Thanks in advance, it will save me a lot of time.

http://i.imgur.com/nSY6Gu9.gifv


I'm sure the same effect can easily be accomplished in GIMP, and I can provide the files he is using if necessary, I just don't really have a good workflow.  Specifically, the goal is to easily, quickly, and accurately make a part of the COLOR texture (from a color or section on the MASK texture) be colored with a specific color, like with colorize or colorify, and the MASK texture to have the corresponding part be black.  I am just not getting a lot of the exact detail when I do it manually.

[rich2005]

Not into games, and do not really follow all of that video. However, Lots of screenshots so having to group them:

screenshots 01 https://i.imgur.com/NIsWzOa.jpg

1) I have pulled the channels dock out so that both channels and layers are visible. A right click in the channels dock can duplicate one of them. In this case the green channel, and to make it easier to see, temporarily turned off the R and B channel visibility.

2) That gives a Green Channel Copy

3) The channel is a mask and can be edited just the same as any other layer. In this case turn the layer visibility off and the channel visibility on.

4) Now, with the channel copy active, easy to edit, selection and bucket fill as the PS demo or just paint in corrections.

screenshots 02 https://i.imgur.com/iC1W9Rp.jpg

5) Edited might look like this

6) Now reverse those visibility settings, channel copy off and image layer both active and visibility on.

screenshots 03 https://i.imgur.com/0Ep8GvT.jpg

7) Replacing the masked off area, add a layer, in this case a solid magenta

8) Layer -> Mask -> Add Layer Mask and select the channel option. If needed there is an Invert mask check-box (not required here)

9) Results in the masked area showing.

[Mapsking]

(09-29-2021, 10:33 AM)rich2005 Wrote: Not into games, and do not really follow all of that video. However,   Lots of screenshots so having to group them:

screenshots 01  https://i.imgur.com/NIsWzOa.jpg

1) I have pulled the channels dock out so that both channels and layers are visible. A right click in the channels dock can duplicate one of them. In this case the green channel, and to make it easier to see, temporarily turned off the R and B channel visibility.

2) That gives a Green Channel Copy

3) The channel is a mask and can be edited just the same as any other layer. In this case turn the layer visibility off and the channel visibility on.  

4) Now, with the channel copy active, easy to edit, selection and bucket fill as the PS demo or just paint in corrections.

screenshots 02 https://i.imgur.com/iC1W9Rp.jpg

5) Edited might look like this

6) Now reverse those visibility settings, channel copy  off and image layer both active and visibility on.

screenshots 03  https://i.imgur.com/0Ep8GvT.jpg

7) Replacing the masked off area, add a layer, in this case a solid magenta

8) Layer -> Mask -> Add Layer Mask and select the channel option. If needed there is an Invert mask check-box (not required here)

9) Results in the masked area showing.

Awesome, thank you so much, with a minor issue with step #7, I was able to replicate it, thanks a lot for your time,explanation, and images!