I've tried every single purple hair dye there is (I'm not exaggerating, I've tried
everything from L'Oréal, Schwarzkopf, Syoss, Wella, etc.) and I still haven't found one that won't fade to pale red after two weeks. It's quite frustrating. There used to be one that was pretty good, it was by Poly Brillance and the color was called Tokio Black Purple, but it was a special edition that was only available for about a year
My hair usually looks like this when I use "dark purple" or "plum" and what's it all called, so not really purple, more berry colored or reddish brown (my natural hair color is ash blonde):
my pics