What you need to do is to
not turn the picture to black and white.
Just as a practice to get the concept:
1) Take a colour image, work on a copy
2)
Go into quick-mask mode (key Q); or toggle the two rectangular buttons on the toolbox beneath the colour pickers to use Quick Mask.
3) Grab a brush, making sure that you have black selected as your foreground colour, size and soften it to your needs to 'paint' the area you want to keep colour.
4) Paint an area with it, it should paint in red or another colour, use the eraser to clean it up if necessary.
5) Exit Quick-Mask mode, you should now see the area you painted as a selection with the 'marching ants' (the animated dotted line).
6) Then key Apple-U, or menu Image>Adjustments>Hue/Saturation and pull the Saturation down to -100 to pull all of the colour out of the area.
7) If it does the wrong area, then undo it and invert the selection so it chooses the opposite of what was originally selected. (Shift-Apple-I) or Menu>Select>Inverse
Here's a page of links for further messing with Quick Mask Mode.
http://www.graphic-design.com/Photoshop/tutorials/quickmask.html
(That's just one way to do it though; personally I would do it with a layer mask and an adjustment layer although I'm not sure if I could explain that so easily from scratch).