show list of values and new values

Just for example, in php how can I show a list of colors so the user can select one? Also if needed the user could add a new color. This could be a drop down or whatever. How can I make this dynamic so when the user adds the new color that anywhere I show the list of colors in my web pages the new color would be available for selection for this user and all users following? Kind of confusing, let me know if you need more detail. Thanks, Doug.