I have this article read a while ago that would help you regarding this problem. It is hard how to tell you exactly what to do. Glad I have found this:
Step 1: First open the picture in which you want to have some objects of the image colored while the rest black and white.
Step 2: Select Channel Mixer option in the Layer Menu.
This makes a new layer for Channel Mixer which is shown in layer palette.
Step 3: In the Channel Mixer Dialog box click on the monochrome option to convert the picture into black and white.
Step 4: Now select the brush tool as shown in the figure below. Make sure that the foreground color and background color are set to black and white respectively.
Step 5: Now select the brush size.
Step 6: Using the brush highlight all the objects you want colored. Select the appropriate brush size for small details. (Do as shown below)
Here is the final image.