Set the form's TimerInterval property to some value other than 0 (1000=1sec)

Sub Form_Timer()

If Me!Label!.BackColor = &FFFFFF Then
    Me!Label1.BackColor = &HFF         'Set to Red
Else
    Me!Label1.BackColor = &HFFFFFF        'Set to White
End If

When you want it to quit, set the TimerInterval = 0