Select an area (eg, the skin) with whatever degree of accuracy / simplification you desire, and then fill it with the desired color. Repeat with other areas (eg, dress + hair).
There is a threshold tool in PS. It gives hard B&W images, but it typically is not useful when large areas containing structure need to selected.
The next question is obviously which selection tool to use. For something with smooth curves such as shown in your example, the pen tool is, by far, the best approach. If you know Adobe Illustrator, its pen tool is better than the corresponding tool in Photoshop. For example, you can put paths on separate layers, group them, use spiral and other advanced options, etc. In fact, I have a strong suspicion that your example was constructed in Illustrator.
HTH,
Tom M