By default, Adobe Illustrator sets the view so that all artwork is previewed in color. However, you can choose to display artwork so that only its outlines (or paths) are visible. Viewing artwork without paint attributes speeds up the time it takes to redraw the screen when working with complex artwork.
In Outline mode, linked files are displayed by default as outlined boxes with an X inside. To view the contents of linked files, choose File > Document Setup, choose Artboard at the top of the dialog box, and select Show Images In Outline.