I don't change my background image from not logged in to logged in, so I am not the best person to help you with your question. Maybe someone else can offer a suggestion. It is good for them to know your Moodle and that you are using boost.
Your colors are not the standard Boost colors, so you have probably made some changes in Boost's configuration. However, I think that I understand that you want Moodle to display a different color when a link has been "visited."
As far as I know, Moodle doesn't keep track of links that have been "visited." I could be wrong, but it will take someone with a deeper understanding of Moodle's "link management" to help. Many years ago I reported this kind of problem in
MDL-37383. Also, It might be a browser issue? I am not sure.
Sorry that I can't be of more help. But perhaps my thoughts will "click" for someone else who can provide a solution.