Open the layers palette.

Step-1,  At the bottom of the palette, click the third figure from the right, the half-moon shape. This will add an adjustment to the background layer, this action will bring up an adjustment list. Select Hue/Saturation. Now the Hue/Saturation palette appears.

Step-2,  All the three levers read, Hue=0, Saturation=0, Lightness=0. Move the middle lever, Saturation to the extreme left to read (-100), then Click OK.

The colour was removed, it is now a black and white image. (see Fig 2 )

This is not the best way to convert a colour image to black and white, but it is quick.

Step-3,  From the tools box select a soft brush. Use the size that best performs the work you're trying to do. 
Press 'D' to make sure the black and white colours are selected and start painting.

If you make a mistake by painting more than you wish, it is a simple matter to correct the error. In the Layers palette reverse the white and black colour, ( type the letter 'D' to do that ) and paint over the area you wish to undue and you will restore the original.

So this means that painting with black on top as the active colour  the original colour of the image is restored.

Painting with the white colour as the active one, restores the painted area to black and white again.

The creative possibilities are many.