"To emphasize the difference between app bars and other surfaces, use a secondary color on nearby components such as the floating action button (FAB)."

University of Michigan - Ann Arbor

Through color difference:
- Shadow overlay the top and bottom bars can be differentiated
- Other surfaces can be demarcated

Identifying App Bars

Material Design Documentation

"Top and bottom app bars use an app’s primary color. System bars can use a dark or light variant of the primary color to separate system content from top app bar content."

Top and bottom app bars 

Difference between app bars and other surfaces,

"If the bottom app bar and floating action button are the same color, use shadow or alternate means to create enough distinction between them."

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