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?