Adding my experiences in case they're useful to anyone.
Directions Plum over naturally blonde hair - faded to grey-blue within two washes over virgin hair near the roots, and faded to a lovely collection of blues and greens where I'd previously had highlights (I love the peacock shades, but they're not really work appropriate for me!)
Directions Plum over XXL Pure Purple (permanent bright pink dye) - fades to peacock shades, but only on the previously highlighted sections, the bits that are box dyed but that weren't bleached in the past have faded to pinky-purple.
I've found that it doesn't matter if I add Dark Tulip or Rubine to the Plum, it still fades like Plum regardless of how much pink has been put in the mix! This could be because all my hair except a few inches at the roots has been bleached in the past year though.
