The iOS App Switcher is somewhat bland out of the box, but it doesn’t have to be. A newly released jailbreak tweak dubbed SwitchShades by iOS developer AnthoPak promises to add a splash of color to the App Switcher background based on the application preview card that appears front-and-center at any given time.
As you can see in the screenshot examples above, the background color changes to match the dominant color of the app icon above the application preview cards themselves. This can be changed to colorize based on the application snapshot itself if you wanted to, or you could configure a custom color to display at all times.