There are many threads in this forum detailing methods for wig dying.
Personally, I'd sacrifice accuracy for quality and just go with a 48" wig; Amphigory says the Really Long Wig is thin and it'd be a pain to dye the whole thing purple. It's also not so nice looking on the top/front because it doesn't have a skin top.
My new account is Thia Fayne.
ALA '08 -
Sasori - 0% (planning) | Lavi - 80% (repairs)
Naminé - 100% | Misa (backup) - 100%
Later for '08
Nino - Fire Emblem | Lenna Charlotte Tycoon - FFV
Daenerys Targaryen - A Song of Ice and Fire | More?