I posted on drywall forum with no response so I will try here. I have asked many builders over the years with no satisfactory answer. I have noticed dark areas on drywall where the studs are located. These dark areas can not be removed by cleaning, only by repainting. I can not figure out what causes them. Seen most in older structures. I use to think heat transfer from outside caused dust or moisture to collect in these areas but I have seen them on inside partitions as well. What causes this?
The phonomenon is called 'ghosting'. I've heard folks call it 'brownian motion' but brownian motion has to do only with the way those particles are distributed on the wall by air movement as they speed up when air changes direction.
It is caused by temperature differential. There are subtle differences across wall framing between the wall over studs and that over stud bays. These areas are cooling to the point where water vapor in the home is attracted to them through vapor diffusion. The moisture collects dirt and dust molecules from the air in the home and the wall or ceiling darkens.
It will show up mostly on outer walls, ceilings where there is little or no attic insulation and yes, interior walls when these areas can be cooled enough through air leakage from the attic or basement/crawlspace.
I have the same problem in an apartment complex I do work for. The stud lines show up even though I wash with TSP and undercoat before repainting.
In this casi I know that there are metal studs under the drywall.
Since I don't usually see this anywhere else, I have to assume that it's the temperature differential, and if the contractor had put some type of insulating material between the studs and drywall, it would not look so unsightly.