Hi Mike,
there is no direct way to change the background colour of a cell, however the cell contents of the table can be rendered as HTML and so you could change the column to contain for example a div with a green background color and the text in it. You can use a String Manipulation for that for example.
You might then also want to remove the padding of the cells, which you can do with custom CSS as explained here https://www.knime.com/css-styling
Hope this helps,
Christian