My hair is pink / purple / black, and this is how I'm keeping mine awesome :)
I bleached my hair out almost platinum blonde, then used N'Rage demi-permanent hair colour (you can get it here: http://www.sallybeauty.com/n-rage/SBS-240686,es,pd.html) in Bubblegum Pink and Plum Purple.
The purple is aces good; it's not faded at ALL and I've had it in for over a month. It does tend to tint the pink, and I will say the pink kind of sucks, but I refreshed it with Kool-Aid, believe it or not.
(I used 2 packets Strawberry Lemonade and half a packet of Cherry unsweetened Kool Aid to make my refresher :D)
N'Rage is a damn good brand as far as bright and long lasting colours go. And you don't have to give up washing your hair (too much - I'd say 1 - 2x/wk with regular shampoo or daily with dry shampoo. LUSH makes an awesome dry shampoo that smells like citrus)