Well although using the color balance in Photoshop works like that, I used an overlay blending mode. Basically I create a new layer, fill it with the color I want (or gradient, or whatever) and select overlay from the blending modes list. By choosing that method, I can more easily edit and I don't have to alter the drawing itself.
She's the first character for the Vocaloid 2 Character Vocal Series. Vocaloid is software by Yamaha that creates synthesized vocals by typing in the lyrics and creating the melody. The only part of the vocals that isn't synthesized is the original voice sample that is taken from the original singer to create it. It's actually really, really cool. You should youtube it.