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