0 day dependency means: A work item start date is on the same date as when the dependency work item is due.

When this happens, the dependency line is shown in black, and not red. It's only shown in red when it's greater than 0 day.

It seems that a while ago 0 day dependency was red, but changed to black from customer feedback. Now there is an interest in making this red again.

This suggestion ticket captures interest and feedback around the 0 day dependency line colour. If there are variety of use cases and different needs, a configurable user setting needs to be considered.

This is for the TImeline and Dependency page inside Jira Plan.