Colour of canvas ? As some have already posted, dark colours are good, don't show up grim and dirt to readily. But, IMHO the best part is prevents too much light coming in in the early hours of the morning, thus allowing a nice sleep in, if the birds allow you too.
Regards the tarp, wouldn't leave home without it. When you've camped out and the possums have used your roof for a toilet, you'll quickly understand the benefits of using a tarp. LOL.
It also prevents tree sap from sticking onto your canvas FOREVER. Heaps easier to remove it from the tarp then canvas, keeps full sun off the canvas thus cooler inside, sheds rain better then canvas and acts as double roof when raining thus assuring water proofing under any extreme weather.
IMHO, all CT manufactures should provide such a tarp as does CAMPRITE.