I guess this would fall into the category of cell shading. Sorry the quality is crummy, converting it sort of lowered the quality. I'm terrified to post this because if you're comfortable with photoshop, it's going to be painfully obvious how uncomfortable I am with it. ; Masks? Locking layers? Heh... uh, yeah. I wish I could say I'm just a n00b, but really, I just suck using photoshop.

1) Lay down the base color for the hair. I clean it up at this stage because I usually have this selected throughout the rest of my coloring, just on different layers. That way I don't have to go back and erase my messy coloring on 5+ different layers (note: I put EVERYTHING on different layers; it helps to do that with my particular style of coloring).

2) First layer of shadows.

3) Second layer of shadows; I put these wherever a deep shadow might occur, and also along the tips of the hair.

4) First layer of highlights; I put this picture up to demonstrate that there's really no easy way to do this. I mean, I'm sure there is, I just haven't figured it out yet, so I just do it the hard way. These highlights should go where there's an excess amount of color that doesn't have shadows on it.

5) Done with the first layer of highlights; don't go too crazy with this step unless it's obvious the character is out in bright light.

6) Lowering the opacity to 40%; the color doesn't mesh well with the base color (too pink), but we'll fix that later.

7) Second layer of highlights; I used the same color I used for the first layer of highlights. Do these even more sparingly than the first layer of highlights.

8) Now that we've got all the colors on the hair, you can change the hue of the first layer of highlights to match the palette better. I bumped the hue up to +25.

9) Augh, okay, this is wear my inexperience really shows. There's probably a million other ways to do this better, but this is the way I've come up with on my own. Select the second highlight layer, and add a streak of pure white through the middle of the highlight with the Soft Round brush. We're going for a gradient look here.

10) Deselect the layer, and then use Gaussian Blur (I used a 4.0 px radius).

11) And that's about it!

12) I skipped a step here, but I merge all the hair layers together and adjust the hue/saturation/lightness until it's a little more to my liking. Here I made Yoko's hair a little more red, and lowered the saturation a little so that it wasn't burning my eyes.

All done in Adobe Photoshop CS2.