Hello Mike,
As Hemikiller posted, with a white interior, black was the default color unless a special edition such as the '72 Sprint cars. If your vehicle is equipped with a deluxe interior, then there should be a color-coordinated carpet on the lower portion of the molded door panel you can use to help ID the original color. The Mach 1/Deluxe interior carpet also included embossed carpet runners that were color-coordinated. My Money Pit#1 (Pewter car) has a black interior and carpet with grey runners. Money Pit#2 has a red/Vermilion interior with black carpet and red runners. If you are considering a blue carpet, Ford didn't get too carried away with the naming on this interior color. My interior info states either blue or medium blue. Ford changed the blue carpet part number each year, but the part numbers were marked "MW" (mix with) which meant the dealer could use any of the three-part numbers, and they would match.
Since Auto Custom Carpet (ACC) manufactures most of the repo carpet for the industry, you can check their site for more detailed information. They offer custom colors, and you can request samples to compare against your carpet. https://www.accmats.com/commerce/
Since many different Mustang parts vendors sell this carpet, you can check your favorite site, make some price comparisons, and check for any sales any of them may have