You can change the colour - or indeed any defined formattting of a
cell - by using a routine called by the Workbook_SheetCalculate,
Workbook_SheetSelectionChange or similar events. Just use an IF
statement or any other logical test that results in a change in
formatting.

For example:

If Cells(2,3).Value = <some condition Then
Cells(2,3).Interior.ColorIndex = 20
End If