All users are able to utilize the Priority tag. However, personal tags can only be added by Premium users.
If you have a Premium account, you may create a new from the Tag Menu, which can be accessed by tapping on any of your tasks and choosing the icon. From this menu, choose the 'Add New Tag' option at the bottom of the screen to create your custom tag. You may add as many tags as you wish, as long as long as they do not use the same title and color combination.