Not sure exactly why you are asking, to replicate or follow the wire or curious.
The general answer I can give is:
The old cloth covered wire, Natural color is a light tan, not Tan. The tail light wire you mentioned is Natural with a White tracer. The tracer is a woven-in, or otherwise colored, line or hounds tooth that looks spiralled. Tracers are now used when you run out of solid colors.
The old wire was given a "varnish" type coating and, after time, Natural can be dark yellow, orange or brown appearing.
If Chevy changed to non-cloth covered in your era, it should follow the same visual color. If it is non cloth, Natural would fade differently than mentioned.
In the modern PVC or cross linked insulated wiring harnesses available with new fuse panels that I have used, the Natural color is NOT supplied. The standardized GM colors are used. These colors sometimes do not match the older wiring diagrams, That's why you buy a good kit with GM colors and is also stamped as to function.. Then, when trouble shooting, you refer to their diagram/colors or just read the circuit/function stamped on the wire.
In our old trucks, unless the wire is in a wire loom, wires are easy to follow or buzz out.
Cloth covered wire harnesses, with proper vintage colors, (including Natural) are available from vintage harness makers....at enormous prices.
