You want to have a black and white except for a portion of the photo to preserve color.
For most image modification software: First you must isolate a part of the picture (the one you want to keep in color) by selecting it. Then simply reverse the selection and desaturate it to remove color.
Example:
Under PhotoFiltre:
- Select the object to keep
- Click on selection/invert
- Then select Filter /black and white or other