Dye won't hold, will rub off in your clothes.
I have had BASIC success using rattle-can SEM vinyl dye to darken faded areas. The way I have had success was to open up the end seam and slide the buckle parts completely off (restore as needed while off). You will want to have an upholstery shop available to stitch this back on when finished. DO NOT SEPARATE THE FLOOR-MOUNT ENDS. This will leave the belts in "As Safe As They Were At The Onset" condition once completed. I usually will pull the tags too and have those stitched back on at the same time (take pictures and take notes). When the belts are partially apart, use Oven Cleaner to scrub clean with a stiff brush, pressure wash the belts till clean, then repeat cleaning till you see no more crud coming out. Dry in the sun a minimal of 1-day (longer drying time required if it isn't a sunny summer day). Mask off the floor mounts and shoot LIGHT MIST COATS of vinyl spray. Wet coats will cause a "brittle feeling" of the belts. A person can change colors too, to a degree, but not like red to white or darker to lighter in color but Dark to Darker is possible. Clean any overspray on the floor mounts with lacquer thinner.
Upon edit: See REPLY #13 for more details