Hi Rick,
There seem to be a variety of colours of seatbelts used.
Here is the strap on the seat on the original Aviatik Berg at the Techniks museum Vienna.
Here is a seatbelt ( different design) with other artefacts in the Museum of Flight, Seattle:
Here is MvR's Fok Dr.1 seat from 425/17.
There are only a few small scraps of the covering still attached to the seat. When I saw it, there was a very small piece showing the black leather material which was originally on the seat. You can just make it out on the picture. Of course this was a later aircraft, but it may be the case that they used the same method on the earlier seats. As the truth will most likely never be known, you have some artistic licence. Hope this is of some help?
Finally, this darker seat belt appears on the Aeroconservancy website:
http://www.aeroconservancy.com/seatbelt.htm