ALWAYS READ THE INSTRUCTIONS!!!
Here is a lesson I just learnt the hard way! My mum bought some hair dye, and asked me to touch up her re-growths, she has fairly short hair and I thought it would be such a waste to throw out the remainder, so I decided I would do my re-growths too! Well while my mum's hair turned out quite nice, my once blonde hair is now a very unflattering shade of orange!
Here is my lesson... If I had bothered to read the insrtuctions I would have performed the strand test, I would have left the solution in for the required time, and I would still have blonde hair!