If the subform is being displayed in Datasheet View, as they frequently are,
no formatting of this type is going to change how it's displayed. Formatting
for Datasheets is controlled thru Tools - Options, and apply to all textboxes
in on Datasheets in the database.
What you can do is use Conditional Formatting to trick Access.
In Design View, select the textbox, then goto
Format - Conditional Formatting
Select "Expression Is"
then, in the next box, enter
Not IsNull9[YourTextboxName])
Now select he forecolor you want.and check OK.