  We use MARCS model atmosphere fluxes to compute synthetic colours, bolometric
corrections and reddening coefficients for the Hipparcos/Tycho, Pan-STARRS1,
SkyMapper and JWST systems. Tables and interpolation subroutines are provided
to transform isochrones from the theoretical to various observational planes,
to derive bolometric corrections, synthetic colours and colour-temperature
relations at nearly any given point of the HR diagram for 2600 K < Teff < 8000
K, and different values of reddening in 85 photometric filters. We use absolute
spectrophotometry from the CALSPEC library to show that bolometric fluxes can
be recovered to ~2 percent from bolometric corrections in a single band, when
input stellar parameters are well known for FG dwarfs at various metallicities.
This sole source of uncertainty impacts interferometric effective temperatures
to ~0.5 percent (or 30 K at the solar temperature). Uncertainties are halved
when combining bolometric corrections in more bands, and limited by the
fundamental uncertainty of the current absolute flux scale at 1 percent. Stars
in the RAVE DR5 catalogue are used to validate the quality of our MARCS
synthetic photometry in selected filters across the optical and infrared range.
This investigation shows that extant MARCS synthetic fluxes are able to
reproduce the main features observed in stellar populations across the Galactic
disc.
