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