Hey!
So I was trying to paint only roughness and metalness to my model (no color because it’s already there). I used to do it by deactivating the color checkboxes (highliguhted in red in my screenshot) and it used to work both with force paint all or the paint brush.
I can’t get it work now, it still paints the color regardless .-.
THanks for your help 
Those checkboxes only work for the base layer, but it can still be done with layers:
You need to access the dot menu in the layers menu and turn down the color slider in the channel factors.
It still technically paints the color…but it’s just hidden now.
That helps, thanks for the tip!
A layer only have one opacity channel, shared between colors, metalness and roughness.
That’s why you can’t use this feature with a layer.
The boxes should have green greyed out though, maybe you wouldn’t have missed the warning just above then 
Oh blimey I completely missed the warning, I feel so silly.
It all makes sense now. Thanks a lot for clarifying!
