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 colours.
Select the cells and choose Format - Cells.
On the Numbers tab, select a number format and mark Negative numbers red check box. Click OK.
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 Format code. For example, instead of RED, enter YELLOW. If the new code appears in the list after clicking the Add icon, this is a valid entry.