The S View cover on Galaxy S5 is mainly used to get important notifications without opening the case. To set a custom wallpaper and change S View background color on Galaxy S5, do as follows:
1. Open Settings app and go to the Accessories menu (General >> System >> Accessories)
2. Under S View Cover options, tap the "S View window wallpaper"
3. You can either select from available background colors, or use a custom wallpaper by tapping the Add Wallpaper button on the bottom right.
4. Press Done and the new background/wallpaper is applied.