The darker each patch was, the deeper it sank. The black cloth and the glass pane sank deepest. Franklin didn't publish these results until much later. The white paper absorbed less heat, so it took longer to catch fire than the dark paper. Franklin brought out issues that weren't explained until the 20th century.
White cloth reflects light.
Heat radiation is a lot like light, but not entirely so. Black skin absorbs light. But it reflects heat just the way white skin does. If it didn't, Nigerians would all have to live in Stockholm. Ben Franklin's pane of glass made a similar point.