The 64, 65 and 66 Mustang Electrical Assembly Manuals references two Ford part numbers for V-8 spark plugs wires, one for the left bank, one for the right bank. Those part numbers are for a sub-assembly, four wires and two wire separators making the group. This was for ease of assembly. They "may" have been color coded for identification purposes on the assembly line. NOS spark plug wires are for service and come in a set of 8 (and a coil to distributor wire) under a single Ford part number and as a service set would not have color codes.
Jim