The PCI colors are assigned based on a periodic rotation of colors.

Once you have made the change, you will need to regenerate all of the images based on the update.  To to this,

choose the legend by applying the checkbox to the left, then "Apply".  A new dialog will show the entire list of 17 projects that have this legend assigned.  Choose the specific project(s) you want to update and choose OK.

The server will now delete the previous images that used the old legend and will create all new images based on the new legend and color assignment.