You can not use a format for that.  You would have to code
the condition in some appropriate event and set the form's
timerinterval.  The form's Timer event will then fire at the
specified interval and you can use that to change the
control's color properties.

In general, this is not a good practice because flasing
screens dirve a lot of people nuts.  Besides, you must
disable the timer event to prevent it from interfering
whenever you are editing any VBA procedure.