It's to hide the water stains.
There is apparently no way to seal the windows and various seams so that water will not seep in when it rains. We knew there was water damage when we bought it. The S.O. invested many hours in rebuilding the front end, sealing the corners, sealing the roof, and then putting up new paneling on the interior walls. I swear the first time it rained after he'd put that new nice white paneling up there were water stains around the windows. Heck, I'd swear there were times where all it took was a heavy dew and more water stains would appear.
It has occurred to me that we've managed to put a lot of time and money into something that was supposed to save us the hassle of putting a lot of time and money into building an actual small cabin. At this point, we're both thinking that we should have just done a smaller version of our house. It would have cost a little more, but at least when it was done, it would have been done. With the guest cabin, each time we do something, we're left wondering if we've finally solved the problem. Maybe this time we'll get lucky.