How allow users to select the theme color?

For admins: I would register an option page in wp-admin and would render some kind of color selecting gadget, that when changed, changes the value of an input field, and then render that color in a template in a style block to change the main color.

For registered users, suppose you could do it with a plugin like Extra User Details.

If you need to change it on the fly, you must use JS, and all the elements that need to change have to have some unique classes.