Sometimes, after you assign a material to an object in 3DS Max, you want to go back to the wireframe color. Deleting the material from the material editor slot or assigning an empty material is not going to do the trick. Removing a material from an object is only possible through MAXScript. To do this you have to select your object object, open the MAXScript Listener (keyboard shortcut F11) and type:

$.material = NULL

If you want to check what material are you using on an object you simply type:

$.material

which will produce an output like this:

05 - Default:Car_Paint_Material__mi

See you later.