After a pedagogical introduction to the main concepts of synthetic photometry , colours and bolometric corrections in the Johnson-Cousins , 2MASS , and HST-ACS/WFC3 photometric systems are generated from MARCS synthetic fluxes for various [ Fe / H ] and [ \alpha / Fe ] combinations , and virtually any value of E ( B - V ) \lesssim 0.7 . The successes and failures of model fluxes in reproducing the observed magnitudes are highlighted . Overall , extant synthetic fluxes predict quite realistic broad-band colours and bolometric corrections , especially at optical and longer wavelengths : further improvements of the predictions for the blue and ultraviolet spectral regions await the use of hydrodynamic models where the microturbulent velocity is not treated as a free parameter . We show how the morphology of the colour-magnitude diagram ( CMD ) changes for different values of [ Fe / H ] and [ \alpha / Fe ] ; in particular , how suitable colour combinations can easily discriminate between red giant branch and lower main sequence populations with different [ \alpha / Fe ] , due to the concomitant loops and swings in the CMD . We also provide computer programs to produce tables of synthetic bolometric corrections as well as routines to interpolate in them . These colour– T _ { eff } –metallicity relations may be used to convert isochrones for different chemical compositions to various bandpasses assuming observed reddening values , thus bypassing the standard assumption of a constant colour excess for stars of different spectral type . We also show how such an assumption can lead to significant systematic errors . The MARCS transformations presented in this study promise to provide important constraints on our understanding of the multiple stellar populations found in globular clusters ( e.g. , the colours of lower main sequence stars are predicted to depend strongly on [ \alpha / Fe ] ) and of those located towards/in the Galactic bulge .