This Plot Details tab is only available when you vary color By Points, then you choose Color Options of Indexing or Increment from.
Origin 2019 begins a phase-out of the Color List tab. To edit a color list in Origin 2019 or later, go to the color control for a particular element -- Symbol Edge Color, Line Color, Pattern Fill Color, etc. -- and click the list control (downward-pointing arrow) beside that element. On the By Points tab (see image above), click the pencil icon at the right end of the color list and this will open the Increment Editor, giving you access to the color list.
For information on customizing the color list, see these topics: