What objects visible to the naked eye at night have the narrowest spectrum of visible light reaching the earth. Put another way, Which objects are most visible as a specific color or colors rather than white or whitish light?
...the narrowest spectrum of visible light...
is tough because most process that make electromagnetic radiation either result in broad continua or multiple emission lines.
I originally wanted to mention microwave masers (see How do we know that comets definitely mase and not just fluoresce? What is it about 18-cm lines that indicates that is really masing per se?) since radio astronomers often refer to these frequencies as generalized "light" as well, but then noticed the "visible" there.
If you can ignore the other lines somehow, then the bright red Hα light from the Orion nebula is a good contender for an answer. I won't include an image because they're usually false color images and/or made from combining images through narrow filters including infrared, but here are some examples of its spectrum below.
Here's a spectrum from stars.astro.illinois.edu's Orion Nebula
Above is the visual spectrum of the Orion Nebula from a wavelength just to the long side of the Hydrogen-Alpha line at 6563 Angstroms to just shortward of H-Delta at 4101 Angstroms. (The wavelength scale is in nanometers; multiply by 10 to get Angstroms.) Energetic ultraviolet starlight (most of which comes from the hot class O6 star Theta-1 Orionis C) ionizes a portion of the molecular cloud, that is, it strips electrons from the nebula's atoms, which are mostly hydrogen. When the charged ions recombine with the free electrons, the energy is given back up as light in the form of emission lines. The Hydrogen Alpha line helps give much of the nebula its reddish color. From the strong neutral helium line at 5876 Angstroms we can get the abundacnce of helium relative to that of hydrogen.
Additional "forbidden" emission lines (forbidden under simplified theory, but strong in nebular spectra) are produced by collisions between electrons and ions, which elevate the ions' electrons to excited states from which they descend, again releasing energy. The strong forbidden lines (indicated by square brackets) of doubly ionized oxygen ([O III}) just left of give a greenish glow to the nebula's central portion, while those of ionized nitrogn that bracket H-Alpha add to the general red color of the outer portions of the nebula. Not shown, to the right of the H-Alpha and [N II] lines are a pair of strong forbidden lines of ionized sulfer ([S II]), while to the left of H-Delta is a pair of strong forbidden llines of ionized oxygen ([O II]), as seen in the spectra of the planetary nebulae BV-1, the Ring Nebula, NGC 7009, and IC 418. Weak forbidden lines of neutral nitrogen show up near dead center.
Photo: University of Illinois Prairie Observatory; spectrum: Okayama Astrophysical Observatory, NOAJ.
By Jim Kaler.
From Bob Stephens and Ralph Megna's Spectroscopy at Megnaritaville
In late January 2009, Bob Stephens and Ralph Megna devoted several nights to collecting spectra of various subjects, including asteroids, nebula, stars and galaxies, using the 14-inch Meade LX200R PopeScope.
One of their targets was M42, the great nebula in Orion (see visible light image here). Spectra from their observations were processed and analyzed in VSPEC. The results included the data chart (bottom of graphic below), a synthesized color spectrum (immediately above chart) and a comparison spectrum obtained at a professional observatory (top).
This analysis shows why M42 looks slightly greenish when viewed through a moderately-sized scope in a dark sky - much of the light energy comes from the emissions of O-III and hydrogen-beta. The red light from hydrogen-alpha and sulfur is more difficult for the human eye to detect, but is obvious in time-exposure images.
(Bob Stephens and Ralph Megna with the PopeScope and SBIG DSS-7 spectrascope with ST-7 camera in January 2009)