Permanent or semi-permanent hair colours can be safely removed with a mixture of developer and shampoo, as long as your hair is not already super damaged. Developer--a product used by professional hairdressers in combination with light hair colours that lightens the colour of hair, making it more receptive to light hair dyes--contains peroxide, so if your hair has been bleached too harshly, you may want to leave your hair alone. Using developer with shampoo is recommended because shampoo will help your hair retain moisture.
Mix equal parts developer and shampoo in your mixing bowl.
Wet your hair in the shower.
Lather up your shampoo/developer mixture in your hair.
Allow the mixture to develop for 10 to 15 minutes.
Rinse the mixture thoroughly.
Finish with a leave-in conditioner. Spray or squirt leave-in conditioner in your hair and do not rinse. This will protect your hair from the peroxide in the developer.
The higher the volume of the developer, the more hair dye lift you will get. Level 30 and level 40 are very strong, and contain more hydrogen peroxide.
Do not leave the developer in your hair for longer than 25 minutes.