To change any cell background color, use the following code in the After Record Processed event.
$record["FieldName_css"]='background:red;';
You can also change the background color of a cell when the mouse hovers over it:
$record["FieldName_hovercss"]='background:yellow;';
See also:
•Change the row background color
•Grid Row JavaScript API: row.fieldCell()
•About Grid Row JavaScript API