This plugin adds a div in the user-chosen top corner of the site (default is right) that will open up on mouseover to a few buttons.
If not logged in- there's a row of four buttons that choose an animated (going from the default background color to the chosen color) background, four that choose a static background (just the chosen color), two that choose either the left or right corner for the div to be in, and a button that puts the background back to default. The color of the div is chosen by the admin in the plugin settings, and the colors that can be chosen are also picked by the admin. It is linked to browser cookies, because you can't use a key if you're not logged in.
If logged in- there's a button that says, "Set options." Clicking that will open a dialog for you to choose what color the div is (including its border), what color each button is for, and which corner the div appears in. You can also choose what each button is named. Essentially, you have much more power. AND- it is linked to a single key.
Colors persist through pages, by the way, so you don't have to set the color again after navigating to some other part of the site.