Hey Erik, here I am again to bother you 
so… I did some (accidental) testing.
After moving some uv’s to another channel I forgot to apply the new uv-channels to some of the meshes.
In this case they did not have a 4th (3rd ID in ue4) channel.
And that resulted that the green channel only showed one pixel (assuming this) of the material applied over the whole green channel.
so I did a rebuild lighting:
as expected, the whole room became green-ish. (loving that particular color of green btw)
so I applied the actual material to it, which turns the green to brown:
and rebuild the lighting:
the reason this was happening was as you explained that a certain color applied to the green channel after bouncing around got very dominant, but not only that.
It’s uv-size (density) also had something to do with it.
The reason I was moving uv-channels was to make sure that the green channel texture’s density is always the same on every mesh. (which is why I had so many material instances, to get as close as I could)
So now all the meshes their green channels density is identical on every mesh.
And I did another rebuild:
and bam! uniform color everywhere.
No need to tweak the materials, or use dynamic lighting or a grayscale version.
I just had to make sure the density was uniform on each vertex color’s layer.
Can I get 1/2 (or even just 1/4th) of “accepted answer” for this?
Also, not sure if you are coming to gdc… if not ill ask chance to give you a hug for your help!