Henna is not temporary
Henna is a permanent hair dye, the only way to remove it is to grow it out, therefore its always recommended to do a strand test first.
It will be messy and it does stain, but as they say you can’t make an omelette without breaking some eggs! Just make sure you wear gloves and cover surfaces, and all will be fine. Its also wise to use a balm or Vaseline around your hairline as a protective shield for your face.
Henna will not lighten your hair tone
Henna is a chemically-free all natural hair dye. It doesn’t contain chemicals, or bleaching ingredients that would lighten your hair.
Coverage of highlights/roots
If you already have highlights or roots, it may not even out the tone, so the highlights will still show up, this however could make for a very pretty result.
Using chemical dyes after henna
As long as you use 100% pure henna powder, then you can use chemical dyes/treatments as you normally do. You won’t have to wait months to do your chemical treatments/dyes as the hair dresser may tell you. (Keep in mind, that the hair industry is run by major companies that push all types of chemical products. They are in no way trained, or specialising in henna, and natural hair care for the most part). You should give your hair a break in between treatments. Roughly about 1-2 weeks.