I believe that the Plum color occurs because of different silicon content in the steel and/or different part temperature and salt bluing solution temperatures.
Wikipedia has an interesting
Bluing Article that talks about "Pluming" or "Browning" which involves controlled "Red" rust as part of the process.
Other articles indicate that it does shift in color with age...
Marc
__________________
Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.