another way (which i find easier, but as planet said there are several ways...)
Duplicate the pic on another layer...desaturate it....using the polygon lasso tool, select/highlight by drawing round the bit you want in colour...delete
(tip: if you don't do this to the windows then background, you can see through the window, will remain black and white instead of colour)
basically deleteing the black and white car from the 2nd layer revealing the colour car behind it on layer 1...
one I done a while ago...
How do you "frame" the pics with the black border though often wondered that as it always makes the pic look soooo much better with a black border