To modify a material, select the node under Materials in the Browser. A material's Type parameter places the material into one of six categories: Electric Conductor (Bulk Conductivity), Electric Conductor (Bulk Resistivity), Dielectric, Permanent Magnet, Nonlinear Magnetic and Perfect Electric Conductor. The Type parameter determines the visibility of the other parameters in the property grid and also plays a role in overlap processing. When creating a new material set the Type parameter to the choice which best matches the type of material you are creating.
To modify the color of a material, click the color box in the Browser or the material's property grid to bring up a color picker tool.