That last one of the Islander is an exception (reminds me of our old skydiving Islander which was like that on the inside too!) but notice most of these shots are taken in winter, and it's the de-icing agent which sticks to the aircraft and picks up the dirt. Initially orange, it seems to change to grey after a short while, as you see here. And of course one of the problems if you have a cold base (like Finnair in Helsinki for example) is not only do you use tons of deicer but you can't do much washing in their climate either because the water will just freeze everywhere.

Of course, the old Braniff had the best idea of how to handle orange deicer!

Braniff