Hi there!
You are supposed to find Header color option within the Preset tab of your Template Options.

If you are using predefined Header then you may use the following code in your custom CSS to change that:
#sp-header:before {
background-image: linear-gradient(90deg, #dc3545 0%, #6973fd 100%) !important;
}
You can modify the color within the above code as per your wish!
Note: Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
You will find related info in the template documentation too:
https://www.joomshaper.com/documentation/joomla-templates/opus/template-customization#Opus-Header-Color
Hope this helps!
Best Regards