"VHT Penetrating Color Dye for vinyl and carpeting is a one-step process that restores or changes the color of vinyl or short nap carpeting found in today's boats, watercraft and automobiles.
It is ideal for use on stiff or pliable vinyl, simulated leather and plastic. VHT Penetrating Color Dye is not a paint but a true dye which permanently changes the color of the substrate.
It is extremely resistant to cracking or fading. Ideal for use on auto interiors, motor-cycle seats, boat interiors, snowmobiles, personal watercraft, household and office furniture." from:
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...or you could use shoe leather cream polish if you are trying to go darker in color.