I think you'd make a selection of the object you wish to remain in color, then inverse the select. Then bring down the saturation all the way down, and if you like change the brightness/contrast a bit. If the options are grayed out, then go to Image > Mode > RGB Color.
Well, there are several ways to get that effect. I would suggest looking at this Photoshop Tutorial site to find which way would be the best option for your own personal skill level.
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