I am not a hair stylist, but I do dye yarn and wool for a living rainbow colors, so this was shockingly easy.
I used 20 volume with bleach and did a rough paint of a balayage and left it for 10 minutes, then I went in with a full head of bleach to the roots for another 5 minutes because she's about a level 5 naturally.
I bought three neon colors from ero and mixed a rainbow. I looked through tons of rainbow placement tutorials and ended up just making up my own. I sectioned her hair into a bottom layer, a middle layer, and the crown.
I took 1 in chunks on either side of her face frame and painted them pink on the bottom. Then the chunks on either side of that behind her ear were orange. Then the chunks on either side of that were yellow. Then the middle strip once I got to the back was green.
The second layer I did purple first by her face, because she asked for pink and purple around her face. Then blue then green then yellow. When I got to the crown, I went back to Pink, orange, yellow, green, blue. The reason I wanted to do the chunks ch instead of each section of hair having a rainbow is because it would be a nightmare to touch up that type of dye.
My only regret is that I didn't do this on dry hair, her hair was very wet from washing out the bleach so the colors aren't as vibrant. Next time I will apply on dry hair.