  We have calculated synthetic spectra for typical chemical element mixtures
(i.e., a standard alpha-enhanced distribution, and distributions displaying CN
and ONa anticorrelations) found in the various subpopulations harboured by
Galactic globular clusters. From the spectra we have determined bolometric
corrections to the standard Johnson-Cousins and Stroemgren filters, and finally
predicted colours. These bolometric corrections and colour-transformations,
coupled to our theoretical isochrones with the appropriate chemical
composition, provide a complete and self-consistent set of theoretical
predictions for the effect of abundance variations on the observed cluster CMD.
CNO abundance variations affect mainly wavelengths shorter than 400 nm, due to
the arise of molecular absorption bands in cooler atmospheres. As a
consequence, colour and magnitude changes are largest in the blue filters,
independently of using broad or intermediate bandpasses. Colour-magnitude
diagrams involving uvy and UB filters (and their various possible colour
combinations) are thus the ones best suited to infer photometrically the
presence of multiple stellar generations in individual clusters. They are
particularly sensitive to variations in the N abundance, with the largest
variations affecting the Red Giant Branch (RGB) and lower Main Sequence (MS).
BVI diagrams are expected to display multiple sequences only if the different
populations are characterized by variations of the C+N+O sum and helium
abundance, that lead to changes in luminosity and effective temperature, but
leave the flux distribution above 400 nm practically unaffected. A variation of
just the helium abundance, up to the level we investigate here, affects
exclusively the interior structure of stars, and is largely irrelevant for the
atmospheric structure and the resulting flux distribution in the whole
wavelength range spanned by our analysis.
