What is the developer strength? Usually semi-permanents are around 10 vol. I believe?

Hi, Thank you for your interest in iroiro. iroiro is a direct dye and does not require a developer. We do recommend lifting your original color first and the level we recommend is 9-10.

does this work on natural hair colour - dishwater blond?

Yes. It worked on my brown hair so blonde should work great.

How long does it stay? 2 washes? 5? 10?

This is not a wash out, it's semi permanent. In my hair the pink is initially really bright when first dyed (on bleached hair) and fades more pastel, but 5 months later it's still there going strong & never goes away. Pink lasts forever for me, while the neon yellow fades drastically over a month, & non existent afer 2 months. All hair is different though. This is by far my favorite brand ever.

My hair already bright dark pink will it work

If it’s already dark pink then this won’t come out neon it would probably just be darker pink. My daughter has dark brown hair so I bleached it out before coloring with this.