Not sure I understand, but all I did was open the pic in PS (in my case I ran gimp because Im having a problem with PS at the moment). I duplicated the layer, desaturated it (to make it black and white), and did a layer mask (white for full opacity) on it. Then I used a Black brush (to remove the layer mask) and roughtly ran over the car with the brush on 100% opacity. So at this point it looks pretty crappy, there are parts of the background that are colored along with the car and it is in no shape to show to anyone. This is when you zoom in and use a really small brush to clean up the details. On your layer mask white makes it black and white, Black paints the color back in. I make my brush as small as I can while still using 100% opacity. Now the car is starting to look great, but there is still a little barrier between the car and background where the background is colored, creating a halo around the car. Now to fix this, you select your white brush (to make the picture B&W), set the brush width nice and small, and set the brush opacity to 40% or so. This will give you a nice smooth blend and is great for covering minor flaws.
After all is said and done, select all, copy merged and paste as a new layer. Save and enjoy.
I used to try and lasso the object I didnt want B&W, invert and desaturate, which I still do from time to time, but I always layer mask so that i can paint color back in or remove as I need. and a 50%or lower opacity brush is your friend for making it look more natural and flowing.
I hope this has answered your question, if not please let me know, or if you are still having issues let me know and I'd be willing to try and walk you through it in even more detail.