If you need to color the actual data points which are above or below the confidence interval, then you will need to do it with a calculated field as you mentioned.
However, you can look at filling the region above or below the confidence interval with some color that will help you separate out the data points as shown in the image below.
Thanks for the reply Hari.
I will have a look at doing this, but I think I would like to be able to have the colour of the data points reflect whether they are within or outside the CI.
Looking like I may have to manually calculate upper and lower CI in my data source and return a TRUE/FALSE in a calculated field. I thought there may be a way of accessing Tableau's CI calculation and using it.
Well, Mitchell, for us to refer the internal calculation that Tableau is doing, we will need a visible and accessible name for it. Tableau internally might be using a name but has not made it available for us to use. So, till such time, we have to create our own calculated fields and proceed.