"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 

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

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."

Bottom app bar and floating action button

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

University of Michigan - Ann Arbor

Arizona State University

The way color is applied to the top and bottom app bars:
- Helps users identify them 
- Understand their relationship to surrounding elements

Top and Bottom App Bars

Material Design Documentation

Identifying App Bars

When an app’s top or bottom app bar color is the same color as the background color:
- Blend together
- Placing emphasis on an app’s content instead of its structure

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Related

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