In reply to The Lemming:
A slightly more nuanced approach is to use a 'Chanel Mixer' (Photoshop term, but I'm sure other tools have something similar) which allows you to control the contributions of red, green and blue in the final images. A bit like having colour filters and B/W film.
Silver Efex seems to be popular - although I've never used it in anger. It's hard to find the free version, but I did manage eventually (last year, I was more interested in some other of the Nik tools). It's also rather clunky to use since it was designed as a plugin, but with a crude exe wrapper - the only way to open a file is to drag-and-drop it onto the exe file.