Traditionally the 'paint' on canvased roofs was made with white lead, then from the late fifties, early sixties paint manufacturers started replacing white lead with titanium dioxide. They did this because TiO2 was much less reactive, easier to handle, cheaper, brighter (brilliant white) and doesn't fry babies brains. This was generally seen as a Good Thing™ except it has caused endless argument from people who don't understand their paint chemistry.