Unless your theme has a setting to change the colour, you will have to add some custom CSS to do that. However, if you made some changes that affected the navigation at the start, perhaps try to reverse everything you did previously that might have affected it. Other than that, you may need to add custom CSS.
Regarding moving the nav menu below the header area, there are two ways to do that:
- create a child theme and modify the code
- add custom CSS (which I can paste some code for you here)
As a side note, I just noticed that you are using the Soledad commercial theme (and using a child theme). If you have recently purchased that theme, you should still have support from the developers that can help solve the navigation colour issue and they may have a better solution to relocate the main menu below your header.
I know that theme from before, but their coding methods for it will be something they are more familiar with than me.
Best Regards,
Andre