Walls and Ceilings - dark spots on ceiling

The house is 32 years old and for most of that time we have had some discoloration on a bedroom ceiling. The bedroom is on the top floor and the discoloration is along the outside wall. It appears to coincide with the rafters.
It feels cooler than the surrounding areas. I would like to find the cause so that it could be fixed. I suspect it may hve something to do with the cellulose insulation in the attic, but am wondering if anyone has had to address a similar problem.

Any discoloration, especially dark, is mostly caused by water damage. You probably have a leak in your roof due to a flashing issue. Also if the insulation is packed to deep into your roof (where the wall meets the roof) and you live in a cold climate, then you will get ice damage.