The most extreme example in common use is probably low-pressure sodium lamps (those intense yellow-orange streetlights), which emit almost exclusively at two very close wavelengths, 589.0 and 589.6 nm.

Yep -- if you take photographs under these lights (e.g. night street scenes), you essentially get tinted monochrome photographs. Under an almost-single-wavelength source of light there are no colors, only illumination intensities.