How to get platinum highlights on colored hair?

I have naturally blonde hair kind of dishwater color. I dyed it black and continued every month or so redying for about six months. I havent dyed my hair in now almost 4 months my roots hair grown out about an inch maybe more and the color has faded to a dark reddish brown. I have gone from blonde to brown and back alot and this time i dont want to bleach my whole head so im going for very bold highlights kind of punkish looking my hair is shoulder length a little longer very healthy. I bought a tipping cap and needle, Ion Color Brilliance Powder lightener and 40 developer(i know its a little harsh) and i want platinum blonde highlights do you think it will turn out that blonde or will i still get the orange brassy color? i plan to do the middle of the section then the ends leaving that for about 30 mins then doing the roots that are blonde and letting that sit for 15 mins. so in total letting it process 45 mins