I work for an agency and have always specifically produced my wireframe documents in B&W primarily because they can printed/photocopied monochrome without loosing any vital information that may have been conveyed using colour, but also because clients can't get distracted by the colour of any elements and pull focus away from discussion of the primary outcomes of the document e.g. layout, functions, content.
However don't get me wrong there are lots of times that I've thought adding colour would have been semantically helpful and enhance the visual qualities of the document, but I've resisted. Recently I see a lot more wireframes being produced which rely on colour which of course if the document is exclusively viewed on screen is perfectly fine, what's your opinion and do you produce your wireframe documents in colour or not?