I was given to believe that white paint on the upper fuselage as opposed to natural metal (a la American) was an advantage in reducing cabin air conditioning loads and/or cabin temperatures due to reflection of sunlight. and that it all stemmed from a BOAC (ah! remember them!) experiment in the 1950s on Equatorial routes, when air con was poor and aircraft tended to be unpainted. Certainly enough benefit to offset the extra weight of the paint. Makes you wonder about liveries with dark upper fuselages (eg BMI), which sem to offer the worst of both worlds.
On a related topic I seem to recall that when the BA "tailfins of the world" livery was introduced some years ago the tails were done in pre-printed adhesive vinyls rather than paint to both reduce downtime and also facilitate changes. In which case why is it taking so many years to strip them off and replace them with the current BA standard (which is, after all, just one of those tailfin colours).