In Joomla! you can setup different Styles for each installed Template. The change on the Joomla! Frontend can be done e.g. with menu items.

In this example, I show the creation of 2 Template Styles and their assignment to menu items.

Create new Template Styles

Extensions → Templates → select Protostar → Copy

As an example, the default Protostar Style can be renamed to "protostar - blue". In the advanced settings, the template color is #0088cc.

Name a copy of this "protostar - red" and set the Template Color to red e.g. #cc0044.

Menu items with Template Styles

In the desired Joomla! menu add a new entry, e.g. Single post with menu title "Protostar Red" and corresponding Template Style.

Same with the "Protostar Blue" Style.

A simple test on Joomla! Frontend show how it works.

Final Notes

With this steps you can install, create and assign many Templates & Styles. With the menu items you can assign all as you wish.

These modifications were made and tested with Joomla! 3.8.13.