You can apply materials to model elements in any of the following ways:
In the Family Editor, when creating or modifying a component, you can use family type parameters to apply a different material to each piece of geometry in the component.
To apply materials to system families elements, see Applying a Material by Element Parameter.
In a project, you can select a model element in a view, and use element properties to apply a material.
In a project, you can use the Paint tool to apply a material to selected faces of model elements. See Applying a Material to the Face of an Element.