Oh my gosh I have exactly the same problem!!!
My bleached blond hair is taking the henna well and looking lovely, but my virgin roots which are a darker ashy blond will not take anything!
It is driving me nuts as I had decided to go natural, or more precisely to stop bleaching and lightening (even with natural lightener like honey and lemon), but I do not know what to do with my roots. Or with my lengths.
Please tell me if you find a solution to this!