If you need to change the material after the model is created you either need to loop through all the faces of your meshes, or replace the material in the material library that is created when you load a collada or 3ds file (since several mesh can share a single material).
I've only tried the first option, kind of like this for each mesh you
want to change:
var material:ColorMaterial=new ColorMaterial(0x000000);
for each (var i:Face in myMesh.faces) {
i.material=material;
}
/David
