Once the task editor is open, you have two ways to pick a category. For the first option, swipe left or right at the top of the screen. You will be taken through the different color categories and their names will appear as you swipe. For the second option, tap the button that reads the name of the currently selected category. The different categories will be shown at the bottom of the screen. Select the one you want and now the task will be assigned to that category.
How do I add a task to a category?