You can color drawing objects—AutoShapes, text boxes, WordArt, and objects on the drawing canvas—with fill effects such as gradients.
If you're not sure what these types of drawing objects are or how to insert them, check out the course Add graphics and keep them where you want them. It provides explanations, examples, and practice sessions that familiarize you with their use.
In the meantime, if you have one of these types of objects in your document and want to add a fill, first select it. Then on the Drawing toolbar, click the arrow next to the Fill Color button to open the Fill Effects dialog box. Use the tabs to add patterns, gradients, textures, and other effects.