For best results, do not go beyond three shades of your current hair level in either direction.
Count the levels between your current color and your desired color; for instance, if you have a level 3 hair color (dark brown), and you want a level 6 (dark blond) there are three levels of difference between the hair, so you should use a 30 level developer.
If there are only one or two levels of difference between the original and the desired color, then use the 20 level developer. The 20 and 30 level developers are only used if you are trying to make your hair lighter.
If you want to darken your hair use the 10 level developer.
For best results, do not wash your hair for 24 hours after dyeing.