Paste this into the worksheet module that you're using.
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Cells.Count = 1 Then
Select Case Target
Target.Interior.ColorIndex = 6
Target.Interior.ColorIndex = 41
Target.Interior.ColorIndex = xlNone
This also allows more options (say you want it green if they enter 3 in the future or something) because you can just add another case to the Select Case statement.
If you want to change the colors, the best way I know of finding out the indices is record a macro of turning a cell a specific color and then go check the code to find out what number it was.