The text with the Photograph linked to below is
"a fantastic photo taken by F/Sgt David Walter, 467 Sqn, of the day when a Lanc crew and a B-17 crew decided to see who could fly on the least amount of engines! In this view all four engines visible are feathered. ... UK ... 467 Sqn"
Plenty of exhaust staining visible also.
In the photograph that started this thread, is it possible that the white finish to the Lancaster was some sort of temporary "white-wash" used perhaps for an Arctic exercise and that this has just washed off where it was applied to areas previously contaminated by exhaust?
Avro Lancaster (Image Ref: A12243H) | Flickr - Photo Sharing!