Open the Selective.jpg file. This image is of a beautiful set of flowers. However, we want to turn the image black and white and isolate the one main flower with preserved colors. To do this go to Layer>New Adjustment Layer>Black and White>Ok. This will create a new layer above the background layer. The layer will have a small white box in it. This is called a Layer Mask. Masks help the reveal or conceal parts of an image. In this case the white mask is revealing a black and white overlay.
To conceal the mask, we need to paint with a color that is opposite of white (Black). Use a hard, round brush. Make sure the foreground color is set to black. Paint over the large flower to conceal the black and white layer. If you accidentally hide too much of the mask, simply switch to white and paint to reveal the mask. Save.
Mask Tip: White reveals. Black conceals. Use a soft brush for subtle lines. Hard brushes will have defined lines. You can change the opacity of a brush to allow you to make the mask appear to fade.