well basically if your hair is completely white it means it has pigment (melanin) whatsoever, when you have coloured it yourself you have packed thousands of colour molecules back into your hair making it appear coloured!
still with me?
the only way to remove this colour is by bleach, when bleaching there is certain levels you go through to lift the colour out, depending on the depth and tone of which you are now.
if you wanted to completely go back to white using bleach it would be a very tricky procedure due to the fact that if the bleach is strong and your hair is dark it would need to be reapplied several times, not only risking the condition of your hair but could result in an uneven colour result.
trying to explain this without any technical jargon.
i would advise you to have a colour stripper applied and a colour lighter than what you are now then to leave your hair a while and repeat the process.
it will never return to a natural looking white due to the colours you have applied but if you can get your colour lighter than it is, maybe adding highlights you could let the colour grow out as you wont have such a noticible difference between the colour now and your natural colour.
however if you was to go back to your natural white do you think you would like it as you have coloured it for so many years??
hope this helps and hope i havnt rambled to much.
xx