I am attempting to make this as easy as possible because I've seen harder ways of doing this.


1) Open an image in photoshop then duplicate the layer.

2) On the new layer go to Layer > New Adjustment Layer > Channel Mixer... Click OK, then check the box that says Monochrome.
The image should be in black and white. Also, there should already be a layer mask there.

3) Get a basic, round, black brush and brush the parts you want to be colored.

4) Switch the brush to white to erase the color.





 
And there you go. This can also be done with any selection tool just as long as you fill the selection with black.

 Ex.
Click Here
And Here