Tom:

In most zip cord, one of the wires is perfectly round, and the other is 
either square, or has a distinct ridge molded into the insulation.
I forget which corresponds to which color, but with that one question 
answered by someone with a working pair of eyes, your problem is solved.
Failing that, put a small piece of tape on the red wire, (again found with 
someone with eyes), every four feet or so all along the spool. Then after it 
is rewound, you will have a reliable marker as long as that spool lasts.
Hope these help.