I just went through the exhaustive process of putting pink/purple in my hair so I thought I would share my wisdom. The following is not a how-to but rather tips for achieving and maintaining mermaid hair. I am not a professional. This is based on experience and research.
Firstly, if you have the money, HIRE A PROFESSIONAL.
Be prepared for the whole process to take at least two days (less if you don’t want to sleep, I guess.)
Have someone on hand to fetch more bleach/dye. I’ve learned not to dump things until I am 100% happy with my hair when dry.
Consider color theory when deciding on your color. I don’t use blue because I can only get my hair to yellow (yellow + blue = green). I try to use opaque colors so I’ve never tried pastels. I don’t think I can get my hair light enough.
To avoid splashing dye all over your bathroom, sit down in the shower first and then rinse your hair.
To remove stray dye from your skin, scrub with non-gel toothpaste.
Wash your hair with color safe, salon-quality hair products. I use Redken for damaged hair.
Try not to use heat products on your hair. It will damage it further. If you must (and I usually must) use heat protective spray/lotion.
Wash your hair as little as possible. Hard to do if you ever leave your house during the summer (it’s so hot here it’s like showering in sweat). Two times a week is good, one time a week is better. Your hair will adjust to this but it’s really kind of a process. Dry shampoo is your best friend. Also, I literally plan my week around the day I shampoo my head.
Protect your hair from chlorine and direct sunlight.