Back when I finally got the chance to do a dye test on my hair--and watched it completely dissolve into nothing under just the bleach--I learned that blues and purples are the hardest colours to maintain in one's hair, most especially when bleaching is involved. The reason for it, as I recall, was something about the dye not being able to firmly latch onto the hair. No matter how long you leave the stuff in place before washing, it will wash out rapidly. Greens apparently manage to hold on longer than anything else, but I don't know if it's because that colour holds itself to the hair better or not.

For the hair dressers I've met who manage to maintain blue and purple dye jobs, they've only told me that it's a lot of work to keep the colour. Which (combined with how my hair simply goes into unbeing at the first drop of bleach) more or less shattered my hopes of having fun coloured hair. It's why I ended up doing the henna--because it didn't require bleaching and it faded out without a dye-line.

Once you get your normal colour back, you may want to try just adding the colour on that way. It may not come out as bright, but it may hold on longer. Also, Manic Panic colours apparently wash out faster than most, depending on the tone.

(Ask Brian if he remembers the pep band trip to Fairbanks, and Marty Woodrow, and how Marty's dye job quite literally went down the drain every time he took a shower)