If you want to learn how to do selective desaturation in Photoshop CS4, you should watch this video. Go to File and choose Open. Open the file you want. Go to Image, select Adjustments, and then Replace Color. Select the second droplet or the droplet with a plus sign. Increase Fuzziness, say 106 but you need to decrease Saturation. Set Saturation to -100. Click on the image except for the selected part that you want to have a color. Decrease Fuzziness to know the parts that you need to click on. Just keep clicking until all that you can see is black and white except for the selected object. You can compare the final image to the original one by clicking the Layer thumbnail icon.
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