Quick lesson in how they work.
Colour Strippers
E.g. Effasor and any bleach based mixes. These contains bleaching chemicals and strips colour (both natural and artificial) from the hair. It does cause some damage in the same way any application of lightener does but adding Olaplex to your mix should limit the damage.
You may also find some degree of patchiness depending on colour build up, especially if the client has been using box colours, but obviously not in this case.
Colour Reducers/Removers
Affinage Eraser, Rusk Elimin8, Blondie Rae etc.
These shrink the permanent colour molecules which are removed from the hair when rinsing. That's why you have to rinse the hair for at least five minutes and it also helps if you actively agitate the hair rather than just leaving the shower head flowing over it. Depending on amount of colour build up, you will probably need to re-apply 2-3 times.
The end result will not be the client's original natural colour though, because of the peroxide used in the original tinting process will have naturally lightened the hair a little, even if they had a darker tint applied!
Also, always apply another oxidising tint afterwards. If you don't do this immediately, any few remaining tint molecules will re-oxydise and the hair will still look coloured (albeit with less colour than previously so it might look a bit translucent).
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If someone wants to lighten their hair that has been tinted quite dark and/or with quite a build up of tint, I'd generally go for a colour reducer and charge accordingly as you'd need to set aside enough time to complete several processes. You might be able to use a normal tint + peroxide mix to achieve the desired outcome or depending on how light they want to go, you might still need to use a bleach mix and apply highlights or consider using a balayage technique (as this is generally quicker).
You can theoretically go from black to blonde in one day with little damage but that depends entirely on the state of the hair and you should always charge a realistic price for your work. After all, if the client goes down the highlighting route instead, it will take several sessions and the costs will still add up.
Hope that helps?