Select your subject
Now it's time to work on the top layer.
Click the top layer in the Layer Palette, and use your favorite selection tool to trace the section that you want to appear in color. For this photo, I'm going to make just the pink petals of the flower appear in color, so I'll use the Magnetic Lasso tool, which you can find in the sixth cubby on the left side of the screen.
To use the Magnetic Lasso tool, click on an edge of the petal and then move along it a little at a time. The tool will automatically "snap" to the flower's edge without any clicking, but you can click when you want to force the tool to lock to a particular point, such as around a tricky curve. When you get all the way around, double-click and the petals will be selected.
When you're done isolating the petals, choose Select- Inverse from the menu. Press the [Delete] key to remove everything but the area that is in color. Immediately, the black-and-white image from the bottom layer should show through. Choose Select-Deselect to finish it off.
You should now have a gray-scale image with a single element in color. Save your new photo -- you're done.