In that case, you can do the conditional formatting in the AfterUpdate event
of the control and in the Current event of the form. You will need to do it
in both places.
Example:
Select Case Me.txtMyTextbox
Case < 10
Me.txtMyTextbox.BackColor = 16777215
Case < 20
Me.txtMyTextbox.BackColor = 255
Case Else
Me.txtMyTextbox.BackColor = 8421440
End Select
You can get the numbers for the colors by going to the BackColor item on the
Format tab of the control's Properties sheet and clicking the ... button.
Select the color you want and see what number gets placed into the box.