I know I’m probably jumping in here late, but I’m doing some research on resistors for a project I’m planning next year. The "Vishay Mills" is on my short list.
The older NOS "Mills" resistors are black in color.
The newer "Vishay Mills" resistors are brown in color.
The only difference I have been able to determine are that the older black "Mills" have the silver-plated copper alloy end-caps.
The newer "Vishay Mills" brown have just copper alloy end-caps (no silver). This would seem to coincide with salectric’s feedback that the older black Mills have more of a fast/clean sound in the high frequencies and lean bass.
I would personally go with the newer brown resistors for overall tonal balance unless you are really after that "silver" signature that pushes the upper mids / low highs -- or if you need to replace resistors in an old circuit that uses original Mills and need to match sonic signature.
http://www.vishay.com/docs/31801/mra.pdf
These are the specs for the new Vishay Mills MRA resistors.