how would i get Katy Perrys pink hair

Katy's hair was a darkish golden blonde before the pink (most likely from bleaching out the black dye.) You can see the gold at her roots on the  last two pics. The first pic is edited I think. A mix of pink and violet will get you a similar shade over a dark blonde base.

and i think some ladies have gotten this cooler-toned pink by toning with purple shampoo.  i wish i could remember who, so i could link you!!

What is your base color right now?

It looks like her last root retouch didn't lift well, they're more gold. It looks like her base goes from gold, to a band of pale yellow down to her browline/ears, then it's gold again. It will be easier to get the dusky color she has on her ends, you would only need a gold/yellow base. Then I would apply some Cupcake Pink with just a speck of a violet or purple (Pravana Violet works great for stuff like this). I'm not sure how much to dilute it or if you would need to dilute it at all, just try a couple test strands, one neat, and maybe one 50/50 with conditioner to gauge it.

and i think some ladies have gotten this cooler-toned pink by toning with purple shampoo.  i wish i could remember who, so i could link you!!

I used to get this exact pink when I would leave Joico's violet shampoo over faded Cupcake Pink--I called it dusky rose!

You will need to get pretty much all of the orange tones out, though, or you'll go more salmony than dusky.

Such a pretty color!

and i think some ladies have gotten this cooler-toned pink by toning with purple shampoo.  i wish i could remember who, so i could link you!!

I used to get this exact pink when I would leave Joico's violet shampoo over faded Cupcake Pink--I called it dusky rose!

You will need to get pretty much all of the orange tones out, though, or you'll go more salmony than dusky.

HAH! yay, intemp! i knew i read it somewhere!!