SketchUp’s make component tool turns a bunch of entities in model space into a single component. For example, picture a block of wood in your shop. As a single block of wood it is a component. But you can break that block of wood down into a series of 6 faces and a bunch of edges (model space). Once model entities are turned into components they can not be directly modified in the rest of model space. You must first double click the component to open the edit mode. You will know if you are in edit component mode due to the dashed line boundary box that surrounds the component. It is best to turn all of your parts into components once they reach the shape you desire.
Once your parts are turned into components you can quickly modify the appearance of the entire component by using the paint bucket tool. If you want to change the appearance of a single face of your component you must first double click the component to enter edit mode. Then you can manually set an appearance to a single face.
I always recommend using keyboard shortcuts for everything you do in SketchUp. It will greatly increase your efficiency. The keyboard shortcut for the make component tool is “G”. You will hear me say “G for component” a lot if you watch my project design videos.