code straight out my project, i'm sure you'll figure the rest out wink
Code:
function enable_material_1(){
	if(render_view == view_player[0]){
		return(0);
	}else{
		return(1);
	}
}
function enable_material_2(){
	if(render_view == view_player[1]){
		return(0);
	}else{
		return(1);
	}
}
function enable_material_3(){
	if(render_view == view_player[2]){
		return(0);
	}else{
		return(1);
	}
}
create material events for each possible view combo

Code:
mtl_local[0].event = enable_material_1;
	mtl_local[1].event = enable_material_2;
	mtl_local[2].event = enable_material_3;
	mtl_local[0].flags = ENABLE_VIEW | ENABLE_TREE;
	mtl_local[1].flags = ENABLE_VIEW | ENABLE_TREE;
	mtl_local[2].flags = ENABLE_VIEW | ENABLE_TREE;
set the event per material

Code:
ent_sky[i].material = mtl_local[i];
apply each material per entities as required

Hope this helps