How do I change the color of a column to let the user know it has been modified?

In the Color property of a column, enter the expression IF (column_name <>column_name.Original, RGB(255, 0, 0), RGB(0, 0, 0)).   This will change the color of the column to, in this case, red if the user changes the value and black if the column has not been modified.  The trick here is that column_name <> column_name.Original compares the current column value with the original column value.