Can I set a color on backlog items and tasks?

Yes, there are several ways to set the color of backlog items and tasks. First, there is an optional color field on backlog items and tasks that you can use to directly set a color. To use it in a project, open the project dialog for the project (in the Projects page) and click "Settings" in the upper right corner of the dialog. Then check the "Show color field in backlog items and tasks" checkbox:

The color field will then appear in the popup dialog of backlog items and tasks:

You can also let the color be determined by the tags on the backlog items and tasks. A tag has an option called "Use the tag color for coloring backlog items, tasks, etc.":

If this option is on for a tag, the tag color will be used for coloring the backlog items and tasks that have that tag. If a backlog item or task has multiple tags with the option on, the tag that appears first in the list of tags in the project will determine the color.

You can also let the color be determined by the backlog list or backlog that the backlog items and tasks belong to. To do this, set a color on the backlog list or backlog in the popup dialog for those (click "Edit this backlog" or "Manage backlogs" in the Backlog tab).

Finally, you can let the color be determined by the epics that the backlog items and tasks belong to. A backlog item will inherit the color of its parent epic, if it doesn't itself have a color from somewhere else.