I owned a Weissach back in the early 80's. The interiors did not hold up particularly well and I did end up dying my seats after judicious taping of the red pinstripes. I recall it was labor intensive job.
Anyhow, the folks that I got my dye from are still around and do other things like gas tank paint (for the exterior) which I can speak for as well.
Generally you send them a sample of your leather (snip a piece off the bottom of the seat and send it in. You will get a pretty exact match and the material can be sprayed on or done with a paint brush (a good one, preferably).
Here is the link:
http://www.leatherique.com/products.htm