Example, template Pace – Responsive MultiPurpose Joomla Theme
Our main color is #06d0d8 so, if you want to change color into red (#ff000c)
you need to open file: templates/pace/css/custom.css
(We need notepad++ is free and you can use it, is simple with this app )
So, open custom.css and find color #06d0d8 , find and replace with your color #ff000c and SAVE
Now you can go to your site and clean cache and refresh page
Now, if you are using our Quickinstall, the example this page
To change here into your new color, you need to open your page builder, open your page and in row setting, change color with new color, save and save your page.