Jess, WHY are you doing it the hard way!! lol

Here's an easier way, this is the way I do it:

Select around the area you want to remain in colour, and go to "Selections -> Invert" which will then select the area AROUND the piece you want to remain coloured.

Then go to "Adjust -> Hue And Saturation -> Colorize" and set both hue and saturation to 0.