I was recently taught this one at a seminar (wish I could call it my own). Make a copy layer of the background, then, in LAB color mode, select the lightness channel. This will show you a B&W image. Then change the mode to grayscale. You will now have a B&W image, but the color is still available in history. Use the history brush to paint over the areas where you want the original color to reappear. You might want to turn the opacity down to 50-75% to give it that hand-painted pastel look. The advantages are that you get back the original color with all its variations in color and density, and that there are no rough selection edges to deal with. It's fast, easy, and gives some of the best looking results I have ever seen.