Several ways, but one may be to draw a painted shader map over your people, where you choose the color you want to give them, add this as a child to a surface layer between default shader and parts shader of the object. Perhaps you need a transform shader between painted shader and surface shader set to world position, don't know. This is my theoretical answer, never tested this.
Or you could render a topdown view in low res, paint colors in another layer in Photoshop, save that layer and import as image map shader, and use that the same way as explained before.
EDIT: yes, works.
