The more important question here is why your wife wants darker teak? I know it's a matter of personal preference, but teak is dark enough, especially on the interior. And after you put a finish on it, Cetol or varnish, it will darken a bit more. Actually, some varnishes may add a golden tone to it. You'll probably ruin the appearance of the teak if you try to change it's color. There's nothing more beautiful then the natural grain in teak. So my advice is to bring it out naturally and don't try to change it. Whatever you do, try it on some loose pieces first to make sure it's the finish you really want. It would be a big mess to change it later, if you even could.