All the baby pink dyes from the big brands that came out within the last few years of the pastel trend are absolute crap. They only last a few washes.
What you need to use for long lasting pink is what people who have had crazy hair colors for decades have been using: semi permanent vegetables based dyes like Adore, Directions, Manic Panic etc.
The most permanent pink will be if you take a dark, intense pink in a hue and ondertone you like then dilute it by adding a tiny bit at a time into a big pot of cheap, white conditioner or hair mask until you have the shade you like.
Then you just section and put it on your head like you would regular hair mask then leave it on for as long as you like. They are completely safe to leave on the whole night or day and will not damage your hair, especially not diluted with hair mask. The longer you leave it on, the more intense the color will be.
This is a really cheap and easy way of maintaining baby pink hair. One bottle lasts absolutely ages and you can just use a super cheap mask. The color lasts a lot longer than undiluted baby pink dyes and they don’t damage your hair like regular dyes do.
Ive had mainly pastel pink hair for the last 15 years or so and have tried all the semi permanent dyes. The best one Ive ever used is Adore, its cheap and very pigmented, lasts ages and has a large variation of beautiful pinks.