So highlift and 12% wouldn’t work?No.
If you only use 6% with a highlift tint, it’s going to expose the underlying warmth on the darker hair but it won’t contain enough colour pigment to tone it adequately. You need to bleach and then tone accordingly, depending on what you want to neutralise. Or you could use a normal tint depending on the percentage of grey.
With white hair you have to add colour to it, so usually nothing higher than a level 9.
I don’t think you’ll get a very good coverage using 12% with your 10/0 to be honest. How would you describe this lady’s white areas, does she just have a few greys that need blending away, or does she have lots of white hair that needs proper coverage? If she needs real coverage then you could try 10/0 with 6%, but I wouldn’t let that mixture come into contact with any of her dark hairs, so application will need to be nice and cleanI’d say she’s around a base 6.
So I may use highlift with 12% in the back Where it’s real dark and maybe just 10/0 with 12% where there are whites ?