To single out a subject from within a photograph you can keep your main point of focus in colour and turn the rest of the shot black and white. There are a couple of ways you can do this in Photoshop but one of the most basic and easiest ways is to use the History Brush.
Go black & white
Once you have your image open in Photoshop you need to convert it to black and white. There are several ways you can convert and image to black and white but the quickest way is to go to: Edit>Adjustment>Desaturate (keyboard shortcut Shift+Ctrl+U) .
Click on the history tab in the layers palette. If it's not open go to Window>History to make it visible. In the left column you'll see a brush icon with an arrow bending over at the top and under this icon is a row of boxes. Now click in the box at the side of the previous history state, which in this case is the Make Layer stage, and you'll see a small brush icon appear.
This is the History Brush and a similar one can be found in the right column of the tool bar. Select this tool. Choose a brush size, then paint over the area you want to be in colour. Take care near the edges, although you can paint back in the black & white on the parts you accidentally remove it from if needs be.
Tidy up your image
If you do need to paint some of the black & white detail back in go back to the history tab and click the box next to the stage which says desaturate. This is the stage where the colour was removed so painting with the icon against this point will ensure desaturation is brought back into the areas that shouldn't be coloured. Select a smaller History Brush this time and paint slowly, ensuring colours not removed from places it shouldn't be. Using the magnifying tool to zoom in should help you be more accurate. This can be found in the tools palette or you can click Cntrl and + on your PC's keyboard to zoom in. To zoom back out, just press – instead of +.
Before and after shots: