Do you want to leave only a few objects in your photo with color and make everything else black and white?
This is actually very simple. The idea is to to put two layers of the same photo on top of each other. The one under should be desaturated or made into black and white already while keeping the colors of the one on top.
First you just open your photo in Photoshop and then you right click on its layer and you duplicate it.
Then on the background layer you created you go to Image -> Adjustments -> then Desaturate.
Then with the eraser tool you take out parts on the second layer that you want to have no color.
And that is it! It is that simple. Now you can have your color isolation effect on your photographs!