La RicheDirections Semi-Permanent Hair Colour Dye x2 Pack-Violet (dir)

Will it work on already purple hair ? I die my hair purple with Schwarzkopf but it’s too damaging to my hair so want to switch to semi.

Would this work on darker hair

Do you need to purchase a developer or anything for or apply directly as it comes?

If i wanted to to use this on my kids over summer holiday would it have washed out in time for back to school? 5 weeks?