I only have 1.5" in gray... Sorry
Man made or dyed. Hell, you could use some thin grey fabric or paper. Get creative, if thats your thing and you have a fussy client to satisfy.
Somewhere back a year or so there was discussion about using coloured/patterned rice paper under clear lams to increase the artistic options on a bow. That sort of method would work fine to give grey limbs as well. I did some searching for it but came up empty this morning.Mark