App Designer, picking colors
Just choose three colors you like, either by clicking on the colored rectangle or by entering the color Hexadecimal code. Then decide if you want to use these colors for menus (i.e. home screen, inside screens menu items), bars (i.e. navigation bar and tab bar) or titles (i.e. text).
To help you even more, Jupitee can propose the colors that are right for your app based on your home header screen or your background image!
In advanced mode you can select every color in detail. Just click on the information icon next to each choice, to see an example.