First of all, open the the photo you would like to try this effect on. There is of course not all photos that are suited for this kind of effect but I guess you''ve already understood that.

For this tutorial I chose a photo of a red rose in a blue vase by a window.

Add an Adjustment layer for Hue/Saturtation to your image. Leave all settings as they are, except from Saturtation which you set to -100.

Now select the Mask of the Adjustment layer.

Select a brush of suiting size for your subject.
(Having a bigger brush to paint the center of the surface then selecting a smaller when getting closer to the edges is usually a good idea.)
Finally paint the areas where you would like the color to be visible with a black brush. If you paint something you don't want to be there then simply paint it away with a white brush.

That's it. This is something worth playing around with, trying things like Gaussian blur, Crystallize etc. In some photos setting the Adjustment layer opacity to about 70% can also create an interesting effect