How to utilize color and black & white

Hello,
I have been searching everywhere on how to use color with bkack & white photos with photoshop cs2. i have seen many photos that have a person holding balloons and one of them is red while everything else is black & white. That is just an example. I have also seen the photo of one green apple and everything else is black & white. i would like to create something similar. Your help is greatly appreciated

Create a Hue/Saturation layer mask, use it to desaturate the image and paint the layer mask where you want some colour