There have been remnants of the original material in examples
I have had.  In all cases the material was a thin cotton substance
with traces of the original red colour hidden from exposure to light.
I would say it was a deep red moving towards burgundy.  I have a
Symphonion with original frets and, taking into account the light
fading over the years, the original colour is similar red.

I have seen other colours on music boxes but these have always been
recent replacements.  They didn't use silk or felt, which I have seen
often, but it is not impossible to find a good cotton match in fabric
shops.

Hope this helps.
Mark Buckland