A dark purple (somewhere between indigo and eggplant- I had to google shades of purple). I think I'll be putting it in a bun!
Whether or not this is practical, I'll let you decide. At one of the AS events I attended last year, Chris made a really big deal about not changing who "you" are for a firm. If you say your hair and the way it is is a big part of you, you should try and appear as natural as "you" are so that you can feel as comfortable as possible during the interview.
Also, there's perhaps something to be said about a firm judging you on the way you look. You want to be judged on merits, not looks. If a firm arent going to hire you because of your hair, that may not be the firm you want to work for.
Of course, the legal sector is professional and the practitioners no doubt had a duty to present a "respectable" image to their client, but... some food for thought.