Keep in mind that you are asking about an assembly line's part number which is different than a dealer's service stock number, the "ZZ" in this case. (The documentation for the service number refers back to and says to use the assembly line part number to make, identify and inspect the part, but the dealer orders, and stocks the service number.)
The door grill is shown in the 1969 Mustang Electric Manual (AM0028) pg55, item 3, as two options, C9ZA-18978-CW, standard door trim only, and C9ZA-18978-BW, Mustangs with decor door trim and all Cougars. These are the actual Ford design casting numbers. The 70 manual shows a -EW and -FW for the same use. The difference is most likely a design change (strengthen holes or webbing are the most common). As Scott shows in reply 10, it is not color.
Jim