This will change it from one colour to the other as you move the mouse over the textbox; you'd need to do the same thing on the MouseMove() event of the Detail too, just reversing the colours.
Code:
Private Sub txtYourTextBox_MouseMove()
If Me.txtYourTextBox.BorderColor = vbRed Then
Me.txtYourTextBox.BorderColor = vbBlue
End If
End Sub