Tarpaulin is not the same as sail canvas. Much heavier. A weathered hatch tarp. will turn grey over time, no matter what colour it was when new. My experience only goes back to ships built around 1912 (they were old when I was aboard!) but the rotation of covers was the same during my cadetship (Ben Line) in the 1950s. 2 (sometimes 3, according to the prevailing weather) with the oldest one on the bottom. All turned grey very quickly though. Horrible things, but thats how it was before steel lids came in. Also, as an aside, when it was raining during loading/discharging canvas "tents" were rigged to prevent (hah!) the rain getting into the holds. Never worked unless the derrick plumbed the centre of the hatch, which with union purchase rig it never did. So lots of water gathered and went sloshing into the hold anyway. Even as a humble cadet I could sense stupidity coming from the mind-set of "it's always been done that way".