Combining color and grayscale

This will teach how to select which parts of a photo you want to remain color after you change it to black and white.

Objective

Original

Finished result

Copy and paste the image of your choice onto a new canvas.
Select the layer your image is on in the Layers panel, and Duplicate it. You can do this by hitting Ctrl + J, or by right clicking the layer and selecting "Duplicate Layer...", or by going to Layer >> Duplicate Layer.

To make things easier to follow, I've named the original layer as color, and the duplicate layer as bw (for black and white).
Select layer bw, and Desaturate it. You can do this by hitting Ctrl + Shift + U or by going to Image >> Adjust >> Desaturate.
Using the Polygonal Lasso , make a selection around what you want to be in color. (If you do not see the Polygonal Lasso, hold down the Lasso tool until you see more options.)
Select layer color. Make a Copy (Ctrl + C) of the selection.

Now select layer bw, and Paste (Ctrl + V). Notice that part of the grayscaled image is now in color!
Continue doing this to as many parts as you want, and that's it!

Image credit: Photo by Cat