Oh and is henna a good idea?
henna is a good idea only if you are not planning on changing your hair color until it grows all the way out. Henna reacts very badly with commercial and professional hair color. I'm a stylist and have seen some weird things happen with Henna. Basically the only thing you can do after henna is go darker, and even then the results can be unpredictable. You can not lighten Henna out of the hair.
Henna is so bad for that! If you use henna and then try to dye your hair, it will boil off your head and turn into goo. They should really warn people about that......lol
Where did you hear that? It's not true.
Henna and chemical hair colors can mix badly, but they definitely won't "boil".
The natural conditioners in henna can seal your hair shaft, not allowing dyes to penetrate. That's why you often can't wash or bleach out henna.
If you have hennaed hair and want to use a chemical dye, or have chemically-dyed hair and want to use henna, do a strand test first. Here's how:
1. Get loose hair from your hairbrush, or cut a small lock from a discreet part of your head.
2. Mix the henna or chemical dye you plan on using.
3. Dye the loose hair just like you would your normal hair, following the instructions on the package.
4. Wash out dye/henna, and check results!
You really should be doing this test before dyeing your hair with any
type of dye, as chemical dyes can react with one another. It will also help prevent "oh crap" situations when the color on the box was dark red, but your hair comes out bright orange