Dying grey roots but blonde midlengths/ends

Previously I dyed my mums hair in a blue to green ombré with black roots. (She naturally has black hair but now it grows as a mixture of black and grey). However it's all faded and blended now. Also, she has about an inch of greyish regrowth and a couple inches below that of permanent very dark brown dye (where I had been touching up her roots before).

I decided it would be best if we just got rid of all the colour and started again. I've bought some colour b4 and am going to use it on her hair.

However, the main problem is that I realised the majority of her hair underneath the dye was an orangey yellow, but her roots are grey. While I will dye about 1cm-half an inch of her regrowth the same dark brown/black, the rest of her hair will be blue/green.

I'm worried her midlengths and ends will be a darker blue/green, but nearer the roots (after presoftening?) it will suddenly be bright blue.

I considered bleaching her midlengths and ends to a lighter colour to reduce the colour contrast, but her hair is already quite damaged so I would prefer not to do that.

Help! What should I do? 😛
Thanks! Any help is appreciated 🙂