Should I use bleach 30 vol or 40 vol with olaplex for black hair?

Read the full Olaplex FAQ on the website and download the app if you need further application instructions. Don't increase developer for on-scalp application. Don't use 40 vol for a scalp bleach, even with Olaplex.

Believe it or not, 20/30 vol will lift to about the same result on roots due to the heat from the scalp. The higher volume may work a little bit faster but if 20 vol takes you to light orange then so would the 30 vol, just in a bit less time but with a more brassy result. It is always better to use 20 vol.

Bleach slows down significantly as it works, at around the hour mark it's not lightening much anymore. It depends on how well your hair lightens and how much bleaching it can take. For black, resistant hair you will probably need two bleaches to get light enough for pastel so do 20 vol twice with enough time between bleaching for your hair to recover and assess damage before doing any more processing.

For the box dye, you should use a colour remover first. After that it could take 2-3 bleaches to get it light enough for pastel since you don't have the heat of the scalp. Same comment applies about leaving time between bleaches.

Read the full Olaplex FAQ on the website and download the app if you need further application instructions. Don't increase developer for on-scalp application. Don't use 40 vol for a scalp bleach, even with Olaplex.

Believe it or not, 20/30 vol will lift to about the same result on roots due to the heat from the scalp. The higher volume may work a little bit faster but if 20 vol takes you to light orange then so would the 30 vol, just in a bit less time but with a more brassy result. It is always better to use 20 vol.

Bleach slows down significantly as it works, at around the hour mark it's not lightening much anymore. It depends on how well your hair lightens and how much bleaching it can take. For black, resistant hair you will probably need two bleaches to get light enough for pastel so do 20 vol twice with enough time between bleaching for your hair to recover and assess damage before doing any more processing.

For the box dye, you should use a colour remover first. After that it could take 2-3 bleaches to get it light enough for pastel since you don't have the heat of the scalp. Same comment applies about leaving time between bleaches.

Thank you for the information, I am prepared to do two bleaches and to wait out a week or two before the second bleach. So 20 vol bleach usually would give less brassiness compared to a 30 vol?

Pastel colours need a really pale base and it simply might not be possible to get that light on hair that's been dyed previously, even with Olaplex. Bleach is not very good at lifting dye. Please try a colour remover first of all, such as ColourB4 or Decolour Remover if you're in the UK to get as much of that pigment out as possible. You can use them up to 3 times in quick succession.

It's recommended on this forum to wait a month between bleaches to let the hair normalise and fully assess whether it'll bare the brunt of another bleaching.

You are going to get brassiness when bleaching black hair it is inevitable.
Going from black to pastel will be a slow process if you want to do it safely, at least three months.
The amount of lift you get is hugely dependant on the bleach you use, so use a really good, professional quality bleach and a lower vol peroxide to avoid damage.
Also bleach over coconut oil. (Yes, you can still use coconut oil with Olaplex)
Instead of increasing the level of peroxide, just increase the processing time a little when using Olaplex.