I need tips on dying my hair rainbow?

I'm buying Manic Panic: Amplified Piliarbox Red, Amplified Electric Banana yellow, Amplified Electric Lizard green, Amplified Atomic Turquoise, and Amplified Ultra Violet. For orange I'm mixing the red and yellow. My hair is already bleached because I died it not too long ago. Now, I am dying my bangs and just putting streaks throughout my hair. My bangs were pink, orange, and purple. The purple is practicly gone. The orange looks blonde. And the pink faded ALOT. I was wondering so my hair doesn't fall out, would I be able to put the red over the pink and the purple over what is left of the purple? Would it stay in? I am leaving in the color alot longer then suggested for a longer lasting and brighter color. Also, do I have to dye, wash out, and dry each color at a time? No one tell me now too, it's not permanent and it's just for summer. I just need some help. If you also have any tips on how to keep the color longer that'd be nice. I've only been conditioning my hair, and once in a while use a tiny tiny dot of shampoo so my color would last. But my hair is really thing, so it feels greasy without shampoo. Is there anyway I can use shampoo without losing my color?