There are 2 colors that get applied to the navigation links on all sites. One is the text color, and one is the background color. These are both edited from the Colors section of the Design Tab.
If you rollover the color swatches, you'll see the first one is labeled "Navigation Color 1" and the 2nd one is labeled "Navigation Color 2". On the Main (top) Navigation on larger screens, "Navigation Color 1" is used as the Navigation text, and "Navigation Color 2" is used as the Navigation background. However, on both the Bottom Navigation on larger screens as well as the Navigation on small screens (Mobile), the colors get inverted, or switched.
Simply click the color swatches to select new colors for "Navigation Color 1" and "Navigation Color 2" as pictured below.
So if you are looking to change the background color of the Mobile Navigation Menu, click on "Navigation Color 1" and select your preferred color from the color selector. And if you are looking to change the text color of the Mobile Navigation Menu, click on "Navigation Color 2" and select your preferred color.
Be sure to click "Save" to lock in your changes.