Oh...

the problem with that is the cell may turn red, depending on the data in a number of other cells. Sometimes it may be ok to be empty and other times it may not.

For example;

If cell 'A1' is YES then 'B1' needs to have a number in it and conditional formatting will turn 'B1' red until a number is empty.

If cell 'A1' is NO then 'B1' doesn't need to have a number in it and conditional formatting will not activate and the cell can be left empty