Holmes correctly tells Watson that despite the Hollywood trope of bombs being built with collapsing circuits and color-coded wiring, most bombs are simple, purpose made devices improvised in the mind of the maker, but with very straight forward and simple wiring. Wire colors don't matter because demolitions experts don't use plans set forth in manuals that require specific colored wires to be used for certain purposes. As Holmes puts it, "bomb-makers don't build tests of electrical engineering skills, they build devices that go bang."