Wuuh, thank you. ;_; I get this question a lot so I'll go ahead and paste my usual response here. :')
I bought a Jareth in black off of Arda-wigs.com. For the purple, I bought some purple extensions at Sally's and cut them up in sections so it was the length and width of Eridan's purple. Then I hot glued the purple hair in between two black sections of fabric. This is probably stupid, but then I hot glued it to the inside of the wig. To make it more secure, I also sewed it to the wig once the hot glue dried.
Next, I flipped the wig upside down and then poured almost/not quite boiling water over it til the hair laid back.
Once it dried, I pretty much chopped it up and stuff to the length I wanted it.
Then I took some Got2B Glue and Gel and styled it into spikes and stuff.
I HOPE THAT MADE SENSE. Ha ha, if not, feel free to ask questions. I am by no means a professional.