Finally getting to it!!! 1970 Duster 340

The RIT dye works great on the straps. You just have to be careful with it around the tags. Also, make sure the belts are completely dry before placing back on the seats. Although they may feel dry, They can also bleed over to the seat vinyl if not waiting long enough. Speaking from experience here. LOL.
We dyed the ones on my dads car about 20 years ago, and they still look great, nice and black. The only thing we done wrong was hurried and put them back in car. Not pretty on red vinyl when black transferred to the red. This was way before we completely redone the entire car.