You can pick just one point in a series, and format it differently.
Just click the series on the chart first, and you will see all the
points highlighted. Click again on the point within that series that
you want to re-format. Now you will see just the one point within the
series highlighted. Right-click on the highlighted point, and select
Format Data Point. From there you can change color, shape, weight, etc.
Somewhat precise clicking is required, so watch out for that.