To put colors depending the result of a formula.

Hello,
I'm a very low level user, so please forgive me if I'm asking something stupid.
The question is if it's possible to put the result of a formula, or the fill color of a cell, in one color or other depending of the result.
I mean, if the result is possitive I'd like green color and if it's negative I'd like red.

I suppose that it's quite simple, but ......

Thanks in advance,
Raul.