Hi
hopefully my answer in this previously answered question should help you make the decision:
http://www.theanswerbank.co.uk/Body-and-Soul/Q uestion700678.html
The reason why the colour on your roots worked and not on the mid lengths and ends is because tint will not 'lift' tint. By that i mean it's not like painting a wall where you can just paint over paint. The existing colour on the hair has to be taken into account. And in the case where the colour id dark, it wont lighten in the slightest by applying another tint over it!!
This is defiantly a job for the professionals though!!
Hope this helps