The easiest way is to use Conditional Formatting. In versions of Access up
2007 you can set three custom formats (color, boldness, italics, background,
underline). Next year you'll have more.
Right click the field in Design View and click Conditional Formatting.
If you need to change text colours and other formatting in directly in your
tables, you can do so if they are memo fields and you use RTF, assuming you
are running Access 2007 or later.