Definitely consider doing some advanced colouring courses including a colour correction course as you're really only taught the basics on level 2. You can never have too much education!
Here's a quick overview about removing permanent colour.
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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 colour build up.
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. (Even a low volume peroxide used to deposit colour, will lighten hair)
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 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.
Finally....
Always remember to 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 mount up.
