Select the color (there are a zillion ways to do this), then the inverse selection so that eveyrthing in the photo is selected except the object you want to keep in color. Then you can simply go into your color range or modes and turn it to black and white. I can't recall exactly what they called it on PS7.
The newer versions of PS have B&W tools, but it's just as easy doing it the old fashioned way in my opinion.