Not so much dirt, Lucky, more a lack of Olive Drab paint! The real 'Idiot's Delight' showed MANY patches of bare metal through the camouflage paint, which, although common on the B26, was extreme in this case, and it also displayed variations in shade, as the paint weathered at different levels.
I've always wondered about the weathering on Marauders; was it due to the paint used, possibly a lack of etch-primer, or perhaps just the smooth surface finish of this aircraft that caused this phenomena? I know, as we all probably do, that the paint finish of American aircraft in WW2, particularly Olive Drab, suffered extensively under the British climate (even though Britain doesn't have a climate, only weather!), but it certainly seems more prevalent on the Marauder, over a relatively short period of time. (On average, 9 to 6 months.) Perhaps someone out there has a answer to this?