You cannot Lighten Hair that is currently Coloured with a Semi Permanent or Permanent Hair Colour that is Darker than the Colour you want to be.
For Example
Your Natural Colour
7 Medium Blonde
Your Current Colour
5.53 Chocolate Brown
The Colour you want to be
9.1 Light Ash Blonde
If you apply the Light Ash Blonde over your current Colour and Regrowth, you will end up with
Light Ash Blonde Regrowth and Toned Chocolate Brown ends.
When you want to Lighten Hair that is already Coloured, your Darker Hair Colour must be removed first.