In SketchUp, models would seem dull without some art.Like for example,If you want to make the Titanic,and were done making the hull and superstructure and funnels,without materials it would not look like a Titanic.This is why you can select Materials. You can do this by clicking on the button. When you click this it opens up a sub-application called Materials. From here Default is selected by default but you can select a category and then after that get started. Each category (except the colors one) has around 10 materials in. You can also add your own material by clicking on the button. This lets you upload a texture image or create whatever color you like.