There are many ways to do it. Here is one:
This was just to see how fast one could do a passable job (took 10 seconds).
The method was to use the magic wand with tolerance of 75 (select hands). Then choose feather of 1. Then Invert the selection. Then Desaturate.
If I wanted to do more, I'd use the history brush to lightly touch up the hand that was sort of indistinct on the leg. I'd also use a better method for converting to b/w than desaturate.
Bascially though, many methods will use the history brush. With a tricky area to select to leave red, I would first change everything to b/w then choose the original image as the source of the history brush and then paint the item that is to be red. This works best with the image enlarged.