I was wondering, is there a way to paint on object only with material roughness and metalness without disturbing the color?
For example: I have an object fully painted with nice colors which are blended and now I want to add a little shinyness to some parts but I can’t do that without applying new color because it is assigned to roughness and metalness. Or maybe there is a way and I didn’t find it?
You can add layer and edit in layer option (with that 3 dot call channel factors ). Remove color
Or you can go color shortcut and uncheck color
Yes this is the solution, use channel masking.
However it doesn’t work with layers but in the future it will.
Yes it should work in this case.
But for a more complex scenario (in-between value of the channel factors), you’ll see it fails when merging layers.
It fails for the same reason why channel masking is not available for layers (the reason is that for now color/metalness/roughness are sharing the same per-vertex opacity channel instead of having their own).
Well you already tweak and test about how each layer compress/uncompress for next update? No?
so didn’t merge layer would be the solution in this cases.
And most people who using layer doesn’t merge the layer I think 
unless they are sure that it’s already good.
Or unless it’s same object with same color and shape.
Although my self prefer not to merge if could.
Layer is very useful to prevent wrong doing. So doesn’t need to undone a lot time or redo everything. Delete the wrong layer and make new one without effect other thing in the object.
Or open old project to do some edit.
Thanks guys. It’s exactly what I’m looking for.