The nut in question, 353911-S100, is used on 64 thru 68 Mustangs, convertibles, rear valence and rocker moldings. For 67 and 68 only, 14 are used on the hood mounted turn signal fixtures. It's the same part number and finish so what ever was used in 64 would still be used in 68, Ford would not grant variances in finish color without documentation, which may be part of the TSB instructions used for the vibration complaints. The early cars finish was clear zinc so that should be the finish in 67-68. The seals should be red, although without the original document, it may have been black as a supplier option. (For reference, the nut is identical to 359606-S8 except for the sealer, and 69-70 -S36.)
Jim