I tried "tintex" dye a few years back on my coronet. They orig were blue and wanted black, had to do it a few times. Came out alright I guess, just looked a little "faded". Gotta be something better out there now.
Somebody here said put the complete seat belt in a long tube sock and then Rit dye them. That should protect the buckle. Also, I just Simple Greened mine to clean them( it's amazing how dirty the water got) and then a buddy told me to spray paint them with REM Landau Black to fix some of the overspray from painting the car (done in a barn by some farmers in Iowa). The REM came out perfect. PS; the paint job is now 16 years old and still look new!