Mary Beth
Yes, it can be done.
However, I wouldn't recommend it. It is likely to produce an annoying flickering of the cursor.
There are simpler and better ways (in my opinion) of highlighting the required cells :-
(1) Conditional Formatting
If you want to format the font only:-
Select range C3:C301, go to Format/Conditional Formatting/CellValueIs/LessThanOrEqualTo and type in 0. Set the font colour you want.
If you want to format the cell colour (with or without formatting the font as well):-
FormulaIs/=AND(C1<>"",C1<=0)
or
(2) Cell Custom Format
For example, to have numbers in red that are equal or less than 0 :-
#,##0.00;[Red]-#,##0.00;[Red]0.00
You can use other colours instead of red if you prefer.
Celia