Video editing objects from black and white to colour
This will probably require a technique called rotoscoping, which involves painting a mask on each frame.  You would need to have two layers of video, one which is desaturated (to make the image black and white) and one which is full color.  You would then need to make a mask (a layer which effectively makes a hole in the black and white layer so that the color layer can be seen).  Since the mask may change based on camera movement and actors moving around the scene, you have to update the mask periodically (possibly every frame).

You can simplify the process quite a bit if the objects are stationary in the scene and the camera is stationary on a tripod.  You could also try using object tracking to maintain the placement of the mask roughly, but to get a satisfactory result, you will probably still need some rotoscoping.

After Effects can accomplish what you are looking for.  Any tutorials on rotoscoping and masking in AE would work to demonstrate the finer points of what tools to use to accomplish it.