Pastel, the color belts in your vert, Ford called Parchment. The black belts were probably more commonly found in the standard interior Mustangs, though I have seen color matching belts in standard cars. I remember ordering some color coded belts for some 72 and 73 standard cars where the retractor had quit working. 45 years later and untold numerous changes to these cars and things get harder to verify.
If you check out the Snake oyl site under the webbing listing. you see they along with other seat belt sites refer to the color now as #8000 Ivory. It's the same webbing they use in the belt restoration process. http://www.ssnake-oyl.com/mm5/merchant.mvc?Screen=PROD&Store_Code=Ssnake-Oyl&Product_Code=BWB&Category_Code=SB_WEBBING
If you check out the Snake oyl site under the webbing listing. you see they along with other seat belt sites refer to the color now as #8000 Ivory. It's the same webbing they use in the belt restoration process. http://www.ssnake-oyl.com/mm5/merchant.mvc?Screen=PROD&Store_Code=Ssnake-Oyl&Product_Code=BWB&Category_Code=SB_WEBBING