Hi there,
the easiest way is to create a Header Element:
https://docs.generatepress.com/article/header-element-overview/
You’re not going to add anything to do it so add this to the content:
<!-- merged header -->
Then on the Site Header tab set the header to merged and in the Offset Site Header Height add 130px ( the height of you site header ).
Now you can edit the Navigation colors. Set those colors for your static navigation.
Then the colors you set in the Customizer will apply to the Sticky Navigation only.