Under Modern User Experience (MUX), projects have a different color for their Tile/Banner. The badge of the right corner is generated from the Display mapping of the Overall Status but there is no mapping set for the Tile/Banner.
The Banner/Tile color of the Project is populated from the Banner Color attribute in the Project.
In the Project Blueprint, assign the Banner color in the Project properties or any tabs. Once a project is created from this blueprint (template), the Banner color will appear in the project which can be changed.
For more information please check this link: Clarity: Configure Blueprints for Projects
See also: Projects known issues for Clarity