Nick, as far as i am aware with this process and i am no expert it is the following.
Selectively remove colour from the image allowing you to turn an image into
grayscale, then pull an original colour back into a selected area.
I believe that if the selected area was say a ladies dress that was red & blue for instance it is ok to select that item although it is more than one colour.
If your picture above was greyscale and blue that would be perfect.
Loads of examples
here
Hope this helps