Assignments are color coded by category in the gradebook. Colors are randomly assigned to grading categories on the Edit Grading Categories page. The color displays as a bar above the assignments in the category. The category name is also listed before the assignment name, as pictured below:

Click the Gradebook tab.
Click Columns › Edit Grading Categories.
Enter a name for the category in Create New Category, Named.
Click Add.
Set a percentage in % of Grade.
This is the percentage of the final grade the category is worth.
Add a comment to remind yourself what kind of assignments belong in this category.
Click Save or Save & Return.
You can edit the Category Name, % of Grade, and Comment in the list of categories on the Edit Grading Categories page. Be sure to click Save after editing.