If it is a content control you have to use the OnExit event in the ThisDocument module;
Code:
Private Sub Document_ContentControlOnExit(ByVal CC As ContentControl, Cancel As Boolean)
Select Case CC.Range.Text
Case "Critical"
CC.Range.Shading.BackgroundPatternColorIndex = wdRed
CC.Range.Font.ColorIndex = wdWhite
Case "Medium"
CC.Range.Shading.BackgroundPatternColorIndex = wdYellow
CC.Range.Font.ColorIndex = wdBlack
Case "Low"
CC.Range.Shading.BackgroundPatternColorIndex = wdGreen
CC.Range.Font.ColorIndex = wdWhite
End Select
End Sub