In the Colors section of the Branding Tab of the left tool bar, you can set your brand colors. You can set a Primary Color, Secondary Color, Screen Background Color, and Text Color.
Once those are set, any new component you add to the screens in your app will use those defaults. For example, the App Bar component uses your branding's Primary Color by default (though of course, you can change the color of the component to whatever you like).
When you change the colors in your branding tab, all of the components that are still using those branding colors will be automatically updated all at once.
Your branding colors will be automatically added to the pre-set palette options in the Adalo color picker, along with a lightened and a darkened version of your Primary and Secondary colors. The last two colors of the bottom row in the color picker are set to your selections for text color and screen background color.