Changing a cell to red or green depending on the number

I have been using the formula

=LOOKUP(DB187,{"1a","1b","1c","2a","2b","2c","3a","3b","3c","4a","4b","4c","5a","5b","5c","w"},{11,9,7,17,15,13,23,21,19,29,27,25,35,33,31,3})-LOOKUP(CL187,{"1a","1b","1c","2a","2b","2c","3a","3b","3c","4a","4b","4c","5a","5b","5c","w"},{11,9,7,17,15,13,23,21,19,29,27,25,35,33,31,3})

This formula looks at a childs predicted assessment level and looks at where they are currently and works out the difference. Is there any way of adding something to the above formula that changes the text/cell red if it is a positive number and green if it is a negative number?

I dont even know if it is possible, but would be so good if it was

Cheers guys

Andy