If you have a reference picture of witch Miku, it would be much appreciated, but I would assume she still has her trademark pigtails?
I would recommend buying a wig with clip on pigtails. Normally, the package would come with a short wig and clip-on pigtails, so there should be not need to have to buy them separately. In my opinion, those tend to look the best.
In terms of pigtail length, I'd go for 100cm pigtails, but the longer they are, the harder they'll be to take care of.
For color, there's such a huge range for her hair colors in between green and blue, and I think it's up to your interpretation of her, since I think both look very nice. ^^
I hope that was what you were looking for? xD
Every time I look back on my signature,
I somehow get embarrassed.