Rich : This is a good, and appropriate question. While I can tell you what I have seen, In believe that we need to have confirmation from an owner, or preferably several owners, of known, original K cars to be sure.
First, just to be sure that all are on the "same page" ; there are two wires that attach to the Coil. One is from the Ignition Switch, and attaches to the post marked "BAT" ; the other is from the Distributor (Distributor Primary Wire), and attaches to the post on the Coil marked "DIST". In all cases, the wire from the Ignition appears to terminate with a Black, 90°, push-on connector. The wire from the Distributor is Black, and terminates with a "Ring Connector" for attachment to the Coil. The Ring Connector, has a molded rubber/vinyl insulation around it where the wire is attached to the Ring Connector. The color of this insulation is what is in question.
I have only 2 pictures of supposedly un-restored 1966 K codes (I don't have any of 65's) ; one has obviously had the "coil end" replaced with a crimp-on connector, so that doesn't help. The other has yellow molded insulation around the "wire end" of the Ring Connector. In Mannel's book, although the pictures are in Black & White, the molded insulation is Black. In addition, all of the reproduction Primary Wires currently being made for 1965-67 K Codes, have Black insulation around the "Wire end" of the Ring Connector.
While one can't always trust what is being made as Reproductions, it appears that the answer may be Black.
Hopefully, we can get some input from those with known originals to confirm, or refute this.