Thanks for the feedback Bill,
I totally agree that with weathering a paint scheme of not too disimilar greys can fade to such a degree that the original demarcation is all but lost. And you are spot on about the wing tanks frequently being seen finished in MSG uppers and LACG lowers. In fact I have an in service photo of XT914 plainly showing this.
However, the photos of XT914 on the website www.simply planes.co.uk show the aircraft shortly after its delivery to RAF Brampton being lifted into position (1992 or 1993?). These photos show the paintwork in a new shiny, pristine condition before any influence by the elements had altered the finish. In these shots the upper wing leading edges appear to be the same Barley Grey colour as the fuselage. Furthmore, there is definately no plain demarcation line visible on the wingtanks, the tanks look all one colour.
Photos I shot recently dated 10 Aug 2011 show this is still the case, in fact the rear section of the wingtanks protected from the effects of sun and weather beneath the wings still have a very noticeable sheen as per the original finish, in contrast the forward exposed section, has definately taken on a matt finish like the rest of the upper airframe. Both tanks are finished in an all over LACG colour.
Having physically examined the aircraft again to double check the upper wing colour, I'm still 99% sure they are finished in Barley Grey and not the expected MSG. Admittedly, on overhead photos such as Google Earth they do look darker than the fuselage. However, when viewing the upper surfaces close up on site it can plainly be seen that, over time a heavy film of moss and grime has built up on and become ingrained on the horizontal surfaces giving the impression of a darker upper surface
The photos I have show areas where the airfame was recently jet washed and brushed in an attempt to 'tidy it up'. The areas where the grime was successfully shifted definately appear to be the same weathered grey hue as the fuselage. At the end of the day, as you rightly say Bill its one of those instances of personal inerpretation. Therfore, based on the current photographic & physical evidence I have (unless anyone out there categorically knows how XT914 was finished just prior to being 'gate guarded') I'll stick with all over Barley Grey upper surfaces and LACG lowers...but watch this space!