Munk WordPress Theme provides you ability to customize the color of your Primary Header.
Navigate to Dashboard >> Appearance >> Customize >> Header >> Primary Header
Scroll down below all other options and you will find the section to customize the color for your Primary Header.
Over here, you can change the background color, text color, link color and hover color.
Note: Color customization here does not affects your Primary Navigation. To change color for primary navigation navigate to its Panel in customizer.