how to dynamically add color to each tasks

You can use the onBeforeEventRender event to set custom event color in the Scheduler.

The color can be set either directly using args.data.backColor or via a CSS class (by setting args.data.cssClass).

The source data object is available as args.data.

When loading events, you can add custom properties as needed. It is recommended to store them under the tags property to avoid conflicts with the built-in properties (see also DayPilot.Event.data).