Start with a color image. Press the D key to set your foreground color to black. Press the B key to change to the Brush tool. Go to Options Bar and change from Normal to Color. Pick a soft edge brush from the Brush Picker. Paint over the areas you wish to be in grayscale. Change your Brush size as needed.
This took maybe 2 minutes in Photoshop Elements 3. It is a rough start and needs some clean up for final print.