Far from an expert and I don't particularly have a horse in this race, but I'm looking at EP691 and I'm immediately reminded of the classic 'Saipan Zeros'.
In those two photos they appear to be painted in a unique camouflage, however earlier photos before they were loaded up on USS Copahee don't show this. Several explanations float around ranging from washing off sea spray, sitting out on a dusty runway for months prior to being loaded up and only certain areas being cleaned up etc etc. Nonetheless, there are decals out there being sold with the idea that they are in a special 2-tone camo scheme.
Obviously the conditions in Malta are far different to Saipan, but is it possible that EP691 isn't actually painted in a particular way but the camera merely picks up and highlights dirt and grime due to reflections and lighting? You can see on the 3rd pic the affect that dirt and grime can have on the same paint (I can't see a B&W original so not sure if it's colourised or not). If EP691 is painted in that blue/slate grey/etc paint scheme would dust from the ground kicked up from the propeller and moved around further by maintenance crew's hands and boots cause it to have such a unique scheme?
3 attached images. Click to enlarge.