This is probably not possible but...
I'm looking to update cells in a Google spreadsheet using the conditional formatting tool.(Changes to SERPs rankings)
So when the number in a cell is changed, it recognises if the number was greater or lesser than the current number and then changes the cell colour to red for lesser and green for greater.
So it needs to be able to store the last changed figure in order to make the decision, it doesn't look to be achievable from the conditional formatting options.