This can be done easily by use of "Conditional formatting"
Click in the cell you want to change color. Click on Format. Click on
Conditional formatting.
Excel allows use three "conditions" that you can specify a change in
cell format -- color, font, and border.
Use the drop downs. In your case it will be easy. Just sellect "cell
greater than" and enter "0" as one condition -- then specify your
format change. Click Add
sellect "cell less than" as your second condition and specify the
format. Click ok
Now double-click on the "Format Painter" button (looks like a yellow
paintbrush) and then highlight all other cells that you want formatted
that way.
*** If you use a formula to change the color in a cell, e.g. using
nested If functions, Excel by Default will enter "$" to make the
conditional format unique to that cell. Remove these before using the
format painter to apply the same format to other cells.
Cheers.