There are a TON of ways to do this. Honestly you could probably find as many ways to do this as there are scrappers. But this is my way, and what i consider a very easy way to do this.

To start with edit your photo. get it how you want it, then duplicate it onto a second layer. Make it black and white however you like ( again, tons and tons of ways to do this. )

But, at this point you should now have 2 photo layers. Color on the bottom, and black and white on top. Like this


Now select a soft brush, lower the opacity and flow a bit ( it will be different for what effect you are trying to achieve. the lower you take them, the more subtle your effect will be. With the black and white layer selected, slowly erase away, to reveal the color underneath.



You could do this for whatever area you wanted. But lets take this a step farther. Lets put the slight flush of color back on the cheeks. You really need a low opacity and flow for this, or it will look clownish, but slowly bring back the color.



Now, lets play with another way to do this, to give her some green eyes. Create a new layer, on top of your black and white layer. Select a painbrush, and the color you want to use. i used green. now, set the layers blending mode to color ( you can get some nice effects with different blending modes, on different areas of photos, so i encourage you to experiment with it yourself ) and then paint over the area you are trying to color