I totally get that, colours vary and especially from older pictures etc.
I think this is being fuelled by the canvas sample being a chocolate type brown and even in an old photo you would be able to see a contrast if the canvas was darker than the truck.
At the end of the day you are right and originally there would have been variations between makers and probably units over paint supply etc.
Perhaps my question should rather be can anyone post pictures of restored trucks that have been painted brown that are fitted with a canvas so I can get an idea of what other restorers have done in the past?