Roman, the OD paint was, as has been mentioned, very unstable and prone to fading and abrasion. Depending on the climate, the paint surface could oxidise to a powdery, rough surface, which in turn could become almost polished by the effects of rain, and airstream etc.
Those B17s based in the UK tended to appear slightly different in overall tones, compared to those based in Italy - this being the effects of the different climatic conditions at their home bases. Remember, these aircraft spent their entire lives outside, only going in to a hangar, if there was one, for deep servicing, and then endured the extremes of temperature and winds at high altitude.
The overall appearance of any aircraft finished in OD can vary according to the angle of view and the lighting conditions, more noticeable on large aircraft such as the B17, and, in photographs, by any variation in the original exposure of the film negative (or reversal emulsion in the case of, for example, Kodachrome film), and more so by exposure and processing variations at the printing stage. Add to this the effects of dirt, staining, exhaust heat, and paint-surface wear - from feet, hands, polishing etc, and the variations over an airframe can be very noticeable. Different types of materials - metals, fabrics, compounds etc - will also effect the appearance . Some aircraft were even more prone to the 'battered' look, due to their finished surface skinning, an example being the B26 Marauder.
Finally, some re-painted or repaired and re-painted areas very well might not have been done using U.S. paints. The colours were often mixed locally, to a close approximation and, certainly in the UK, stocks from RAF sources were often used.