I personally do it this way....
1. Open the picture you want in PS
2. "select all" and "copy" the image
3. Open a new canvas and "paste" it on there.
At this point, you have 2 identical images opened up in two different windows
4. Go to one of the windows opened, and do whatever effect you want to it, whether it be black/white, blur, etc.
5. "select all'' and ''copy'' the picture you just did.
6. ''paste'' it on top of the untouched picture that you earlier made a copy of
7. Go to the eraser tool and erase the object(s) in the picture you want to make in full color. I personally zoom in to 400% to make sure that the lines are clean.
That should be it.
Some random example..
I exaggerated the RGB setting in the background, while the car is not touched...
