Vista - Windows Paint program

I imported a picture of a car into Paint and would like to change the color
of the car, but do not know how to do this or if it is possible. I clicked on
the paint bucket and the color I chose, but when I clicked on the car to
change its color, nothing happened. Do I have to somehow outline the car
first or is this just not possible?

Thanks!
P.S. if you know of some other program I can download that will allow me to
do this, please let me know.