Although all of these techniques work, I find the easiest way is to select the painbrush tool, change the blend mode from "normal" to "color," select the size brush you'd like to use (I prefer a medium sized, soft edged brush in this case) and then "paint" the colors away. Everything you "paint" will turn to b&w, so you can "paint" around whatever object it is you'd like to leave in color. Hope that helps.