Use dropdown field to add a class to the body tag
Set up your dropdown so that the Option Labels are default, yellow, green, etc. and the Values are page-project, page-project bg-yellow, page-project bg-green, etc. Then, you can
{% set bodyClass = entry.backgroundColor %}

This will set bodyClass to whatever the value is for the chosen dropdown element, i.e. page-project if you pick the default option.