show color schemes before selection

It is super nice that is so easy to change color schemes from the menus but as a user I don't actually know the colors of those schemes or their assignments; perhaps allowing the user to select them or at least showing them would be beneficial.

No file or test is named. Start at the menus that expose color-scheme selection and trace how schemes are represented in the visualization UI; clarify whether the expected outcome is previews, visible color assignments, or both. The work is done when users can understand each scheme before choosing it.

Written by the indexing model from the issue text.