Bleach > coloured with flamingo > coloured with plum leads to...

Can you give some more details of how you went about getting to this? how faded was the flamingo? How old was the dye? how had it been stored?

Anything you can think of might help!

I bleached a large section of my hair for a tank girl cosplay, admittedly I stupidly forwent the toner and stuck the flamingo on a familar yellow base. I also managed to get the roots a whole lot brighter than the rest, probably a result of having previously used henna.

The pink didn't suit at all, really tacky and inconsistent because of the seriously bright roots. Put on a layer of plum mixed with conditioner and got a gorgeous bright purple.

Over the next few weeks some of my roots reverted to blonde, other bits went bright pink, and the whole thing streaked with blue, which was very pretty until the purple turned green.

Now I have frizzy green mess with the odd purple/blue/pink bit, oh, and some blonde.
Just got out of the shower after trying to strip as much as i could with heat washing, lemon juice and coconut oil.

Was it Directions Plum? The new formula does fade turquoise, so over a yellow base you will get green. What colour do you want ideally?

yeah, it was direction plum, i just wanted purple

I would avoid that particular colour as it is known to do that. Try a different brand, most purples just fade pinky or silvery, some fade bluer but not with the intense green tones that the new formula for Plum has.
Adore Purple rage, crazy Colour Hot purple, Special Effects Pimpin Purple or diluted Deep purple, Pravana Violet will all give you a lovely bright purple.

I also think adore violet gem might work for you as it is very pink based which would help avoid blue/green fade.