Anyone dyed seat belts before?

[Havencrest]

I've got 3 full sets of 3 point retractable belts but they are grey, we are needing black( at 139 a set for new ones,it could be a good cost savings if can be done)--anyone tried it? Found an Acid ( vinegar ) set dye that is supposed to work well, made by Jacquard. Looks promising, and the seatbelts are out of a newer model wrecked expedition--no way I'd do this on 40 year old belts.