Originally Posted by jogr
It's not mold. It's a common phenomenom. The studs are colder because they conduct heat better. I can't explain the physics but dust particulates favorably accumulate on the colder surface producing the stripes. If you burn candles or a fireplace you'll get the stripes faster and darker.
The cold studs cause a tiny amount of condensation where the drywall is against them. This is just enough to trap and collect dust and tiny particles in the air over the years.
There's no real solution but painting the walls regularly will cover up the stripes.