Categories have colours to delineate the different items/columns within the category. Currently, these colours are randomly generated, and change every time you reset/visit the tool.
For users who associate colours with categories, changing the colours may throw them off each time they come to the page. We've received complaints about this, and have modified the behaviour so that the colour generation algorithm is deterministic. In this way, the colour code does not have to be stored anywhere, and the user will always see the same colours for their categories (unless a category is deleted and recreated with the same name).