  Although the use of RGB photometry has exploded in the last decades due to
the advent of high-quality and inexpensive digital cameras equipped with
Bayer-like color filter systems, there is surprisingly no catalogue of bright
stars that can be used for calibration purposes. Since due to their excessive
brightness, accurate enough spectrophotometric measurements of bright stars
typically cannot be performed with modern large telescopes, we have employed
historical 13-color medium-narrow-band photometric data, gathered with quite
reliable photomultipliers, to fit the spectrum of 1346 bright stars using
stellar atmosphere models. This not only constitutes a useful compilation of
bright spectrophotometric standards well spread in the celestial sphere, the
UCM library of spectrophotometric spectra, but allows the generation of a
catalogue of reference RGB magnitudes, with typical random uncertainties $\sim
0.01$ mag. For that purpose, we have defined a new set of spectral sensitivity
curves, computed as the median of 28 sets of empirical sensitivity curves from
the literature, that can be used to establish a standard RGB photometric
system. Conversions between RGB magnitudes computed with any of these sets of
empirical RGB curves and those determined with the new standard photometric
system are provided. Even though particular RGB measurements from single
cameras are not expected to provide extremely accurate photometric data, the
repeatability and multiplicity of observations will allow access to a large
amount of exploitable data in many astronomical fields, such as the detailed
monitoring of light pollution and its impact on the night sky brightness, or
the study of meteors, solar system bodies, variable stars, and transient
objects. In addition, the RGB magnitudes presented here make the sky an
accessible and free laboratory for the calibration of the cameras themselves.
