When on the Task page click on the Manage Tags button on the top right of your screen.
When you do the Tag Management pop up window will open up.
Here you'l be able to Add or Delete any Tags. Remember that if you delete a Tag it will be removed from any Task that had the Tag assigned to it.
To create a new Tag simply enter the name of the Tag in the field.
Select the Tag color by clicking on a color swatch.
Once you’ve happy with the Tag name and the Color you can click on the Save button. Your new Tag will be available to use in the Task edit drawer.