Here you go bubbles-artist! This is my favorite stylist when it comes to rainbow hair. She has a really great ebook about doing rainbow hair like a pro on her website!
Also try blasting your hair with a hair dryer just before rinsing out the colour (you can completely dry out the dye if you want), and using cold water to rinse it. Hot temperatures help open the strand more to better absorb the colour, and the cold water helps seal it in. (I remember hearing that one off Anya Goy, when she was explaining how she stops colours from running into each other, so it's like, got that Salon Professional stamp of approval)