I think I too might follow the approach suggested by NoSparks, but the following should work
Code:
Private Sub txtBillToCust_Exit(ByVal Cancel As MSForms.ReturnBoolean)
'Checks the txtBillToCust field for a blank entry
If Not Len(txtBillToCust.Text) = 8 Then
txtBillToCust.BackColor = &HFF&
MsgBox "Please enter the 8-digit Customer Number"
Cancel = True
txtBillToCust.Text = ""
txtBillToCust.SetFocus
Exit Sub
Else
txtBillToCust.BackColor = &H80000005
End If
End Sub