The most obvious method is to extrude a shape... go to its properties and specify a material for your solid form in the Material parameter under the Materials & Finishes group.
Goto Tools... Paint and you can also paint materials onto individual surfaces like the surface of a pool.
While in the Family Editor go to Family Types... Add Parameter... and specify a new parameter of Type "Material." Now associate your new parameter to the Material parameter in Element Properties of the solid form (click tiny grey button). Once loaded into a project go to the Family's properties... Edit/New. You can now change the material from Family Type to Family Type. This method is very useful if you're expecting the material to change depending on the Family Type.
Extrude a shape... go to its properties and set your material to "By Category." You'll notice a parameter called Subcategory. To add a custom Subcategory go to Settings... Object Styles. After you've created your custom subcategory go back to the elements properties and assign the Subcategory to your solid form. Now when you load your family into a project file you can control materials globally in Settings... Object Styles. This method is very useful if you want to control the palette of your project globally from Object Styles.