Claude, look at Format, Conditional Formatting. This should do what you are looking for.
Jerid
I don't have any of my code samples with me now, but you should look into the Worksheet_Change() routine in Visual Basic. There have been several recent Q&As on this subject lately, but until the recent stuff is restored, I can't refer you to them. Anyway, enter Visual Basic (Tools - Macros - Visual Basic Editor), then View Project Explorer. The worksheet you are working on will be amongst the listings. Right Click on it and select View Code. A new window appears. In the upper-left pulldown select Worksheet. In the upper-right pulldown select Change (NOT Selection Change). What you will see is a subroutine that only has a beginning an end with no code. In the middle you would add the code that says essentially if target.value >= 80 turn the cell red. If target.value <80 turn the cell green. (Sorry, you will have to look up the exact code for this yourself - or record a macro to get the code and insert it into the above formula).
Jerid's answer is obviously much shorter and easier - if it doesn't work with a DDE setup, use what I mentioned.
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
With Selection
If [A1] >= 80 Then
[A1].Font.ColorIndex = 3
End If
If [A1] < 80 Then
[A1].Font.ColorIndex = 0
End If
End With
End Sub
HTH
Jim