 So you want to add different materials to different sub-tools, not a problem. Turn M on and select material. Go to color, fill object and make sure polypaint is enabled. Then just repeat the process for your second object. You're done. If you switch between the two objects now you will see they have two different materials. If you ever need to remove a material on the material, pick flat color, then as usual go to color, fill object, then when you set material back to the default matte cap gray it'll be back to the standard. I hope that helps and as always I'll be having a fantastic day and I'll see you around.