I have just read a very interesting comment on the Solaruk website detailing research on the effect of roof colour on solar water heating, in particular vacuum tubes.
Its pretty obvious when you think about it, wish I had! But basically, a white or reflective roofing which has solar vacuum tubes mounted on it will of course reflect a portion of the energy back on to the solar collector.
The amount of energy reported being 9% was certainly higher than I would have expected but makes the consideration of roof colour very relevant, in particular where considering a new build. I would wonder about the cost benefit of re-roofing, but perhaps a small area with a secondary covering of a reflective roofing surface would make sense.
This does of course apply to flat roofs and only vacuum tube systems, tough perhaps a rack mounted flat panel with the rear suitably coated might have some of these benefits.
The article is here: Improved solar gain with reflective flat roof colouring