If the lifetime of a roof is ~20-30 years, in that time we won't have 'solved' climate change, so exceedingly unlikely that you'll need to repaint it for climate reasons at least.
What I mean is bad localized effects in important areas.
Painting roofs sounds completely benign, but if it's as effective as other geoengineering methods, then there's little reason to think it doesn't have the risks of other methods.
(Personally I think those risks are probably worth taking, given climate tipping points.)
