To change an object's material to your own customized color or photo, click on your Shelf in the "Materials" area and select "User Materials."

Select the subcategory Color and look at the bottom of the Shelf. You will see a button which says "Add a Color." The subcategory Photos also has a button that says "Add a Material."

Click your button of choice and you will be asked to change the color or choose a photo from the library of your device.

For more information on how to add materials to the objects, please watch an excerpt of this video tutorial: