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After a pedagogical introduction to the main concepts of syntheticphotometry, colours and bolometric corrections in the Johnson-Cousins, 2MASS,and HST-ACS/WFC3 photometric systems are generated from MARCS synthetic fluxesfor various [Fe/H] and [alpha/Fe] combinations, and virtually any value ofreddening E(B-V) < 0.7. The successes and failures of model fluxes inreproducing the observed magnitudes are highlighted. Overall, extant syntheticfluxes predict quite realistic broad-band colours and bolometric corrections,especially at optical and longer wavelengths: further improvements of thepredictions for the blue and ultraviolet spectral regions await the use ofhydrodynamic models where the microturbulent velocity is not treated as a freeparameter. We show how the morphology of the colour-magnitude diagram (CMD)changes for different values of [Fe/H] and [alpha/Fe]; in particular, howsuitable colour combinations can easily discriminate between red giant branchand lower main sequence populations with different [alpha/Fe], due to theconcomitant loops and swings in the CMD. We also provide computer programs toproduce tables of synthetic bolometric corrections as well as routines tointerpolate in them. These colour-Teff-metallicity relations may be used toconvert isochrones for different chemical compositions to various bandpassesassuming observed reddening values, thus bypassing the standard assumption of aconstant colour excess for stars of different spectral type. We also show howsuch an assumption can lead to significant systematic errors. The MARCStransformations presented in this study promise to provide importantconstraints on our understanding of the multiple stellar populations found inglobular clusters (e.g., the colours of lower main sequence stars are predictedto depend strongly on [alpha/Fe]) and of those located towards/in the Galacticbulge.

Synthetic Stellar Photometry - I. General considerations and new transformations for broad-band systems