Funny, I had exactly the same impression as the OP. I even made up a theory about it. I thought it was a color filtration layer that would be used by the paper manufacturer to do precise filtration adjustment after all light sensitive layers are coated to correct for batch-to-batch variations of the emulsion. On Fuji paper it surely looks like a surface coat. Maybe I should cut a thin slice and look at it under the microscope to prove myself wrong