You could categorize your tasks by using color labels. In Taskulu you have six color labels and one colorless label.
For editing the title of labels you must Admin or owner in projectthen, click on "Settings" and select "Project Settings", then go to "Color Labels" tab. For activating the edit mode, you just need to click on the title of colors once, then start changing the text. For example, you could change the color "Red" to "Critical".