Once a selection is made you can change it in a couple of different ways
Grow and Similar are both found under the Select menu, or Ctrl/right-click inside a selection to find them in the list.
Grow makes the selection gradually bigger and will select the same colours. Similar picks out all the colours that are similar to the ones already inside your selection, but jumps to any part of your iamge that is similar.
Similar is ideal for slective black and white effects.