Being cynical, I thought it might be to help the bad guys' aim!
It somewhat spoils the whole camouflage idea.
What I have always found strange about the "white cockpit roof" mod is that other air forces did not follow suit. For example, US aircraft in Vietnam, operating in a very hot climate, were camouflaged all over, in darker colours, which would absorb more heat than the RAF's desert camouflage.
Does anyone know the history of this idea?