Colour Reducers versus Colour Strippers
They are different products and achieve slightly different outcomes.
Colour Strippers
E.g. Effasor and any bleach based mixes. These contains bleaching chemicals and bleaches colour (both natural and artificial pigments) from the hair. It does cause some damage in the same way any application of lightener does. You may also find some degree of patchiness depending on the amount of colour build up. The benefit is speed but if there's a lot of colour build up, consider using a colour reducer instead.
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. You can do a quick check for progress by applying a dilute peroxide mix directly onto a section of hair and give it a few minutes to see if it changes back to the previous colour.
The end result will not be the client's original natural colour though, because of the peroxide used in the original tinting process. Also, always apply another oxidising (Demi) tint afterwards. If you don't do this immediately, any few remaining tint molecules will re-oxidise 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 on the condition/type of the hair and the products you are using. Adding Olaplex to the mix will help reduce the damage but Olaplex cannot repair damaged or missing cuticle scales.
Also, 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.
I originally wrote this a couple of years ago but I think it's worth re-posting for newbies.
Hope this helps?