Hey doug,
If you're rendering with Arnold, you can do it super easily with a aiWireframe node, and play with fill and line colors,
If you're rendering with Mental Ray you need to apply a material to your objects, then go on the shadingGroup of the assigned material, while Mental Ray is your current renderer go in the mental ray tab of your shadingGroup then you have a Contours section, turn on Enable Contour Rendering then play with values, if you want a flat cube you can just assign a simple surfaceShader (right click -> assign Favorite Material -> Surface Shader)
Have a nice day,
Mehdi