Colour restoration on roof of '89 Cabrio? - With image

Hey everybody.
my family owns an '89 MK I Cabrio with a white roof. But the colour of the fabric is worn with use, and since I have heard that a altogether "new" roof in mint condition is quite expensive, i was wondering:
what can i do to restore the colour to a slightly more… pristine state?



-Edit
I just checked on the roof up close. it appears that the gray wearing on the roof is in fact ground-in dirt. i managed to dissolve some of it with water and a piece of cloth. but the colour under that looks like an old sneaker. any suggestions for how to restore it?

cant see if its vinyl or mohair, anyhow post below gives some info on how to on a vinyl roof. You may be able to find a similar product.

http://www.vwgolfmk1.org.uk/modules.php?name=Forums&file=viewtopic&t=14539&highlight=hood

im quite sure its vinyl. i'll check out your link