I can see where we can set the background color for a row, but not a column. Will this be something in the works, or will I just have to satisfy myself with changing the color of the text? Or-- even more likely-- am I missing something again? 
There is no Background Color Mapping for columns. However, if there are a set number of values for a given column, eg. Val1, Val2, Val3, you can set a Background Color Translation list in the Table Customizer. If the values have infinite possibilities you can set the background color of the column using HTML in the SQL query, eg:
SELECT Col, CONCAT("<HTML><table width=1000 height=1000><tr><td bgcolor=red>", Col, "</td></tr></table>") as Col2 FROM Table This is a MySQL example where you can embed HTML in the query. You can insert a table with a huge height and width (to guarantee that it is bigger than the cell) with a color of your choice. Hope this helps.
Any chance of explaining how to change the colour of a column. I have a fixed number of rows and want to highlight certain columns to indicate which are/are not editable.
I see in the customiser Background Color List/Column but no documentation as to how to use these.
You can just add another column to the dataset that is the color state (must be an integer). You can hide the column from the user. In the customizer you can set the background color list column to the new column name you just created. So if it was called “colorState” make it the same. Then you can set the number to color translation. If you change the value of the color state column the background color will change.
By the way, this is an Ignition request so in the future post it to the Ignition Design Help forum.