You can format cells with a number format that highlights negative numbers in red. Alternatively, you can define your own number format in which negative numbers are highlighted in other colors.
Select the cells and choose \<emph\>Format - Cells\</emph\>.
On the \<emph\>Numbers\</emph\> tab, select a number format and mark \<emph\>Negative numbers red\</emph\> check box. Click \<emph\>OK\</emph\>.
The cell number format is defined in two parts. The format for positive numbers and zero is defined in front of the semicolon; after the semicolon the formula for negative numbers is defined. You can change the code (RED) under \<emph\>Format code\</emph\>. For example, instead of "RED," enter "YELLOW". If the new code appears in the list after clicking the \<emph\>Add\</emph\> icon, this is a valid entry.