1. Begin by going to SELECT...Color Range. The dialog box to the right will appear. The settings you use will depend on the tonal range of the image you are using, but generally keep "Sampled Colors", "Selection" and "White Matte".

Then select the Color Picker (in red) and click on the part of your image that you wish to retain the color (in this case the red sweatshirt).

Then adjust the Fuzziness slider until the part of the image in the preview window of this box shows a good white/black contrast of the part of the image in which you wish to retain the color.

When you click OK, you should have marching ants around the selected part of the image. Don't worry too much if there are bits of "stray selection" outside the selected area.

2. Create a copy of the layer with the image on it, and click on the Layer Mask icon at the bottom of the layers palette (as shown).

Masks allow you to isolate and protect areas of an image as you apply color changes, filters, or other effects to the rest of the image. When you select part of an image, the area that is not selected is "masked" or protected from editing. You can also use masks for complex image editing such as gradually applying color or filter effects to an image.

3. Now select the original layer (the one without a Layer Mask), and go to IMAGE...Adjustments...Gradient Map and use the drop-down arrow to select the Black-to-White gradient.

4. Go back to the layer with the Layer Mask on it. If any of the areas outside the desired (colored) area still retain some color, pick the Eraser Tool with a large tip and brush over them to convert them to black and white. (You only have to be careful with this when you are near to the edge of the colored area).