An easy way is to:
1- Use lasso tool to select what area you like to have it in color (You have at least 2 good choices in lasso, Regular or Magnet. Try each to see the difference).
2- Select > Inverse. Or [Shift + Ctrl + I].
3- Image > Adjustments > Hue / Saturation [Ctrl + U]. Then reduce the "Saturation" to minus 100 (Moving the level fully to the left).
You are done and have fun.
I used this method for my competition image of month:
L I N K