Not sure I understand your question.
If you add pages or categories to the menu, you can set current color for the menu item at customizer > colors > secondary navigation, the 3rd color is for current menu item: https://docs.generatepress.com/article/global-colors-overview/#pick-colors-for-elements
For example, if you are on the home page, and you have home page in your secondary navigation, the home page menu item will use the color you set in the customizer
It’s the same for primary navigation as well.