
I've seen a few really cool photos where part of the image is in color (such as eyes, flowers, or a shirt) and the rest is in black and white. I set out to discover how to do this myself.
Open your original image. Create a copy of the "background" layer. With the new layer selected, click on the "create a new fill or adjustment layer" icon at the bottom of the layers palette. It's a half-white, half-black circle. Choose "Hue/Saturation" from the pop-up menu, then move the Saturation slider all the way to the left if you want it completely black and white, or you can move it most of the way to the left, to leave a hint of color in the background like I did. I went to about -85. Click OK. Now your image should appear to be black and white. You'll notice that a third layer has been created: that's a layer mask that you just created.
Now to get some color back in there. Make sure that the new layer mask layer is selected, and choose the paintbrush tool with black as your foreground color. Any area that you paint black on this layer will cause the bottom (color) layer to show through. Any area that is white will show your black and white layer. And any area you paint gray will sort of show the color layer. I used the paintbrush to paint our flowers. You might also want to turn down the flow and opacity settings on the paintbrush tool, and lightly add in some color to the face and lips, because you tend to look kind of dead without a little bit of color on your face.
And that's it!