Conditional Formatting in Google Spreadsheet
You can format your spreadsheet's cells on the basis of their contents. You can have condition when you want the cell's background color to turn red if it has a negative number, and to turn green if it has a positive number.
To do the conditional formatting you have to select "Conditional formatting..." from "Format" menu at Menu Bar. Have a look at the following screenshot:
And then you can select the condition that you want to have, you can also have multiple conditions and then click on "save rules".