There is an option inside the colour board to choose a colour selection with an eyedropper.
For instance you could choose the item you wanted to keep with the eyedropper.
Hit the right pointing arrow to enter colour board.
Now at the bottom of the screen choose “outside mask”.
Now go to the Saturation tab at the top, and bring the left had global saturation slider all the way to the bottom.
Hopefully now you will see you chosen item in colour and everything else black and white.
Hope that helps,
Cheers,
Brendan