This is a very simple effect - creating a black and white image from a photograph, and then restoring the colour to certain areas. By doing this, visual emphasis is placed on the coloured image. Its interesting how doing this often gives a completely different feel to the image.

We have broken down this task into 4 simple things to do.

Creating layers

As always, duplicate the original image. We will not alter our original image in any way, allowing us to always go back to the original image if required.
(The keyboard shortcut for this would be to select the layer, press "CTRL + A" to select all, then "CTRL + J" to duplicate the selected area)

Desaturating, or removing colour from our copied layer

We are imaginatively naming our duplicate layer "black and white". When the layer is duplicated, we are going to make it black and white - using the desaturate command:
(Image > Adjust > Desaturate)

Now we have our original colour image being hidden by a black and white copy. In order to recolour some areas, we will hide some parts of the black and white image. We are going to use a layer mask for this.

Adding a layer mask

Layer masks are a great way to cut out parts of images and leave other exposed. They are also risk free, as editing the layer mask will alter what parts of the image are shown - but it will not affect either the colour, or black and white image.

The layer mask we are choosing for this application is "reveal all". This means that as it stands currently, the mask is showing everything - nothing is being hidden.
Note how the black and white layer is selected.

Selectively colourizing your image

Ensure the layer mask layer is the topmost layer in the tools box. Select brush from the tools bar, and the colour black. Decide which parts of your image your image you want to restore colour too.
Then, just zoom in until you are confident you can easily draw in the coloured regions. Draw onto the layer mask, and colour will magically return to the image.
Change brush size depending on how detailed you want to be. And thats it - it is a piece of cake.

Fixing errors

If you draw onto the layer mask using a dark colour, parts of the black and white image will become hidden - exposing the colour image below.
If you make a mistake, and hide a part of the image you didnt mean to, simply select white, and draw on the layer mask.