Normal IRR paint is harmful to canvas. In the early days of IRR there was no special canvas paint, so the instruction was to paint the canvas only at the start of hostilities.
Nowadays there is: "Paint, Finishing, IRR, Emulsion, Matt, NATO Green, Air Drying" it dries in about an hour and should cover the entire canvas. The black bits are in "Paint, Finishing, Polyvinyl Acetate, Emulsion, Black, Air Drying". Note this is not IRR. Other canvas paint also available in Light Stone, Red & White for ambulances markings.