Hello, Dear Ivano!

First of all, you may change background, color and overlay color of sections in Unyson Page Builder. The manual about ho to use Unyson Page Builder you may download here: 

https://drive.google.com/open?id=0B5NKlmlSvX2yYll1a0NOV2hpVms  

It will help you to perform many changes with content and colors on your current website.

If you want to change colors for the custom areas in theme - please use Yellow Pencil plugin. You may download it here: https://drive.google.com/open?id=1QVYd_ztJ_PJVg7uF7D9wa_8vSgdfwrFi

Also please visit this links in order to understand, how this plugin may change your website: https://waspthemes.com/yellow-pencil/?yellow_pencil=true&href=waspthemes.com%2Fyellow-pencil%2Fhome-test%2F&yp_page_id=319&yp_page_type=page&yp_mode=single

https://www.wpexplorer.com/customize-wordpress-theme-design/

Best Regards!