I bleached my roots recently to a pale yellow, not quite inside of a banana skin but pale. I wanted white so used Wella T18 toner and all of my hair went white, except my roots which went to a very very pale yellow but not white. In the space of a week or 2, my roots darkened again despite me not washing it yet. In my haste, I T18'd my roots again tonight and still they won't go white.
It's so frustrating as it happens every time, when I eventually bleach my roots again in a month or so I'll have the same problem again but the hair that has grown out will go white like the rest of it.
Is it possible that my virgin hair is too resistant to tone to white until it's grown out a bit? I just don't understand why my virgin hair won't bleach light enough yet once it's grown out a few months that same hair will tone white.
Yes. I can never get my hair white, unless it's really badly damaged. My roots especially won't go white, but like you, they'll be closer the next time I bleach. When you bleach you likely overlap and so the roots that you've done the time before will be damaged that bit more so it turns white. I seem to just have so much yellow in my hair it's nigh on impossible to get enough out without pushing my hair too far!
The darkening of your roots, especially if you hadn't washed, was likely oil from your scalp making it look darker. Even if it doesn't feel greasy oil will still be present and it could be enough to make your hair look quite a bit darker. The yellow tones will also look more intense then too.
I've never managed to get my roots white either, even though my natural colour is quite light, only the more damaged lengths ever get to anything more than light yellow.