Painting into a Layer Mask to Reveal Image - Photoshop

Open the file you wish to modify Choose: Layers > New Adjustment Layer > Hue / Saturation
The Hue Saturation dialog will open, and a new adjustment layer will appear.
Slide the saturation slider all the way to the left to remove all color. You may want to adjust contrast/brightness if the resulting black and white image doesn't appeal to you.
Open the demonstration diagram (It will open in a separate window so you can continue reading this tutorial)
Click once on the WHITE layer mask generated by the adjustment layer (2)
Tap the letter "D" to set your foreground color to BLACK
Tap the letter "B" to select the Brush Tool, and in the Options bar above set the brush to a medium softness, and an appropriate size for working with your image
Now move into the image window and begin painting over the part of the image you wish to be in color. Painting BLACK into the white adjustment layer's mask will "erase" it to reveal the color subject below.
Change brush sizes as needed. You could also use the Lasso Tool (L) to make a selection, and then Alt/Delete (Option/Delete) to fill the selection with black and accomplish the very same thing.
That's all there is to it.

Method 2: Without Adjustment Layer
Duplicate the background image by dragging it to the "New Layer" button at the bottom of the Layers Palette. Control / Click the new layer's thumbnail (Apple/Click) to select the whole layer Select: Image > Adjustments > Hue / Saturation, and slide the Saturation slider all the way to the left TAP the letter "E" to load the Eraser tool, and begin erasing away image in the image window where you wish to restore color. Once again, adjust the size and softness of the eraser to suit the image under modification.
When complete, Save the file as a PSD layered file, then flatten and use File > Save As... to save the final file.
See: Painting from Color to Grayscale for a similar technique.A Photoshop Elements tutorial is also included.