That's one reason we're pretty sure that the discoloration was caused by the sun, rather than by acid rain. In British Columbia, you wouldn't suspect acid rain anyway. Trapped moisture caused mildew and corrosion. That's why we don't use Nylon or similar materials. Besides, Nylon doesn't hold up very well in sunlight.
Every set is made specifically for the particular make and model of aircraft involved.
Typically, the cover is fitted to go over the wingtip, and then fastens under the wing with a bungie cord, with a loop tied at the end to soft nylon hooks sewn onto the trailing edge of the wing cover. Sunbrella comes in lots of colors. We have selected a light gray for building most of our covers this year. I really like it because it goes with almost any paint scheme and doesn't show dirt. Other colors can be special-ordered, though.