Yes! I would recommend if you have long hair to always get two boxes. That is what I have to do with mine since it is long. I always have found that with just a single box it doesn't cover evenly and I never have enough.
Yes! Just be sure you apply onto semi greasy hair.. maybe after a day of not showering. It will help it stay longer and apply completely even! And of course condition and avoid washing again as long as possible.