Basically, if you have a fairly new version of Photoshop, you can use a Black & White Adjustment Layer, convert the image to black and white to suit your taste, and then paint in black on the layer mask to bring back color in your chosen areas. It's really become quite simple!
If you dont have new version photoshop ... you can copy your photo and convert it to B & W then in editing program place the two photos on top of each other in layers then using erase tool on soft setting and erase the top layer to reveal the coloured (or B&W) layer underneath