While I agree that fly fishermen are full of it (I should know; I am one), I am a believer in not simply dark stuff, but BIG, dark stuff in off-color water. By "off-color," I mean water with less than about a foot of visibility. I won't bother fishing anything muddier than that with flies, first because of the lack of visibility, and also due to the general difficulty of presenting flies where fish can see them in the larger volumes of water that usually come with that level of turbidity. In those conditions, see RB3's post above, or simply say to heck with it, stay home, and live to fight another day. I may be full of it, but I know when to quit.