I think my original approach will work for me eventually (i.e. dividing up my model in the vertex modeler and then regrouping later) Thank you all for your replies - I know now that I'm not just missing something!

I just now tried the shader, and although I could see both colors in the area render preview, they don't export via VectorStyle - I only get one basic color. Same trouble with the image/texture map. I know that VectorStyle probably won't work with texture maps for coloring (because that would defeat the purpose of the vector art output). Hmm.