Do you want the number to be red, or the cell's shading?
If it's the number, use a custom format such as
#,##0;[Red]-#,##0
If it's the shading, use Format> Conditional Formatting
Set 'Cell Value Is' along with 'Less Than' and '0' Then click on the
format button, go to the Patterns tab, pick your color, and OK out of the
dialog box. Copy that formatting to all the cells you want treated that way.
