Just to add to Bernie's sub. If you want to change interior color:
best wises
Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Range("A2:A100")) Is Nothing Then Exit Sub 'Specific range
'Turn off events to keep out of loops
Application.EnableEvents = False
If Target.Value = "x" Then
Target.Value = Chr(252)
Target.Font.Name = "Wingdings"
Target.Interior.ColorIndex = 5 'blue
Else
Target.Font.Name = "Verdana"
Target.Interior.ColorIndex = 0 'white
End If
'Turn events back on to get ready for the next change
Application.EnableEvents = True
End Sub