Cleaning or rejuvenating Z3 storm cover?

[TriumphZ3]

Hi all! I have the additional storm cover for my 1998 Z3 which I use if heavy rain is forecast but after owning it for two years it has bleached to a very light colour, almost grey in colour rather than the hood which is still black. Is there any way of rejuvenating this that won't damage the waterproofing?