I took most of these while walking through Huntington Library in Pasadena.
 young and old
 This is an older technique that is still used in film, but it was quite popular a few years ago in photography (probably overdone!).  The idea is to bring the focus on just one thing with a splash of color.
2 yellow flowers
I did these with "layers" in photoshop.  You copy the color photo to a different layer.
Then convert the copied layer to black and white.
purple flowers
It will look like the entire picture is black and white, but the color is underneath the top layer.
2 ducks
To get the final effect, use the eraser in the black and white layer to remove areas you want the color in the layer below to come through.  

It can take a little bit of time, but the results can be very cool!