Thanks, Mitch: Interesting to know this is not specifically a vinyl problem. I gave the cover a long scrub bath and rinse and there is MUCH less schmutz coming out of the black canvas now but still traces along the hem where the canvas is doubled.
The unusual white-and-silver look is a feature, to me. I was worried about walking to the wrong silver-with-black-tonneau speedster in the grocery-store parking lot.
But hey -- there are still three more covers on ebay. Maybe someone on SOC will get one and let us know how it holds dye.