Start off with a White wig, for the same reason we draw on white paper.
I picked two colours I wanted to use, a Pink and a Blue. I also have hair elastics handy so I can separate out my hair for colour. It's mostly useful to separate the Already done portions.
You can see where this is going. Just start colouring with your Sharpie. You want to get every single strand (but done in clumps) so do it up against a surface that is throw away like a shoe box cover.
I liked holding on to a clump of hair and just rubbing the Sharpie nib and up and down the hair. The colour might need several runs to be even, BUT having it not even looks even cool! Automatic highlights.
I didn't finish the whole wig cause I actually thought it was pretty cool with just the bangs and part of the side as the only color.
Just thought I would throw in this picture for fun. I wore this around work for halloween, but I wanted to straighten it first. Possible but make sure it's on the lowest setting so the fibers dont start melting.