Ok, first of all just use the 'color select' to select the color you want, then copy it, then create a new layer, then paste it.
After that go to 'Layers' -> 'Anchor Layer'.
P.S. If your still getting a new layer, then close GIMP, the re-open it again, (you may have changed a setting), it's happened to me before but don't worry GIMP doesn't save your settings.
Hope this has cleared everything up