Going from brunette to blonde is tricky—brassiness is always an issue, especially for dark brown or black hair. Light brown hair is less likely to take on an orange-y tone.
Choose an ash blonde hair dye to complement and cover any natural orange undertones. Deep condition hair for one hour at least one week before dyeing to repair any cuticle damage and create a more even base for the dye.
Apply the hair dye evenly from root to tip and let it rest for the minimum amount of time listed in the instructions.
Check the processed color of the hair before rinsing off the dye. Separate a small section of hair and scrape off the dye with a gloved fingernail. If it's hard to tell, wipe the hair with a wet rag.
Check the color of the hair in bright natural light. If it's still brassy, or not as light as you'd like, continue processing for the maximum amount of time recommended. Do not go over the maximum processing time or you risk damaging the hair.
Rinse the hair with filtered and deionized water. Mineral and iron deposits in tap water react chemically with the hair dye, causing more brassiness.
If the brassiness remains, apply a blonde toner.
Finish the process with a conditioner to keep hair from drying out.