One problem, my sheet is to analyse student progress and the sheet works out
the levels students have gone up or down in. Therefore the sheet displays
positive and negative integer values including 0 which I need to see (the 0
meaning no progress made from the last assessment). However I want the cell
to be blank when a student has been absent from an assessment so it does not
give me an incorrect progress level.

I want to use a traffic light system below:

-2 or below = red
-1 and 0 = amber
1 or above = green
any blanks = white

So I don't think your suggestion would work here...

Any other ideas?