  The existence of star-to-star light-element abundance variations (multiple
populations, MPs) in massive Galactic and extragalactic star clusters older
than about 2 Gyr is by now well established. Photometry of red giant branch
(RGB) stars has been and still is instrumental in enabling the detection and
characterization of cluster MPs, through the appropriate choices of filters,
colours and colour combinations, that are mainly sensitive to N and --to a
lesser degree-- C stellar surface abundances. An important issue not yet
properly addressed is that the translation of the observed widths of the
cluster RGBs to abundance spreads must account for the effect of the first
dredge-up on the surface chemical patterns, hence on the spectral energy
distributions of stars belonging to the various MPs. We have filled this gap by
studying theoretically the impact of the dredge-up on the predicted widths of
RGBs in clusters hosting MPs. We find that for a given initial range of N
abundances, the first dredge up reduces the predicted RGB widths in N-sensitive
filters compared to the case when its effect on the stellar spectral energy
distributions is not accounted for. This reduction is a strong function of age
and has also a dependence on metallicity. The net effect is an underestimate of
the initial N-abundance ranges from RGB photometry if the first dredge-up is
not accounted for in the modelling, and also the potential determination of
spurious trends of N-abundance spreads with age.
