Ok it wasn't a failure - but he didn't come out the colour I'd expected from the dye packet! He's a cold greeny bluey colour rather than the almost muddy green I'd hoped for...
Current colour next to a doll in the colour he started out in... He's been sealed with Tamiya TS80 to stop any rubbing but I think the plastic has actually gone greeny/blue since I scrubbed his face with hot soapy water and then isopropanol pretty hard to remove the excess pigment that had stuck to it (weirdly this pigment was a browny-red colour - what the heck?!)
Close-up so you can see the uneveness of how the dye took - I think if I'd scrubbed him off more thoroughly the colour would have come out more uniform, but I quite liked this look for the character (think a dirt-covered Labyrinth style goblin)
Rawr!!!
In all I think the acetone idea probably does work - but as MeltedCaramel says I reckon I needed an awful lot more RIT dye in there!