Improve column highlighting in assessment tool workflow

[surchs]

We use colors to give the user visual feedback on what tool is selected and what columns have already been mapped to it.

This should be improved in the following ways

• find a color that is less 💥
• tools that are active should be colored, inactive tools should be white (as now)
• columns that are not mapped to any tool should be white (as now)
• columns that are mapped to a tool should not be white
• if the column is mapped to the currently active (i.e. selected) tool, then the column should be brightly colored, otherwise more muted

Essentially: make it more like the column-categorization workflow above, but with only one color