I just did a quick experiment, creating a consolidated master through the pool and the External Tasks command on the Team Planner formatting ribbon had little effect. All tasks looked the same.

However, when I opened one of the sharer files, I could easily change the external (to that sharer file) task bars.

Actually - strike that. I just experimented again and in the master file, non of the tasks are considered external. I was able to change the color of the bars by changing autoscheduled bar styles, but as no tasks are external, the changing the bar style for external tasks has no effect.