You can tweak screen saver settings on your Android TV to show Chromecast wallpapers or colors when your TV is idle. To do this, head to Home screen > Settings (gear icon) > Device preferences > Screen saver.
You can also make your own photos from the Google Photos app appear as screen savers on your Android TV. To do this, head to the My Apps section on your Android TV and look for an app called Backdrop Daydream. Make sure this app is updated.
Now, head to the Google Home app and tap on the name of your TV. You’ll see an interface that lets you control your Android TV’s volume. On the top right corner, tap on the Settings button (gear icon). Tap Ambient Mode. Here’ you should see three options to choose from — Art Gallery, Experimental, and Google Photos. Tap on the Google Photos option to set your photos as screensavers.