  This is the third paper of a series reporting the results from the POPSTAR
evolutionary synthesis models. The main goal of this work is to present and
discuss the synthetic photometric properties of Single Stellar Populations
(SSPs) resulting from our POPSTAR code. Colours in the Johnson and SDSS
systems, halpha and hbeta luminosities and equivalent widths, and ionising
region size, have been computed for a wide range of metallicity (Z = 0.0001 -
0.05) and age (0.1 Myr to 20 Gyr). We demonstrate the importance of the
contribution of emission lines to broader-band photometry when characterising
stellar populations, through the presentation of both contaminated and
non-contaminated colours (in both the Johnson and SDSS systems). The tabulated
colours include stellar and nebular components, in addition to line emission.
The main application of these models is the determination of physical
properties of a given young ionising cluster, when only photometric
observations are available; for an isolated star forming region, the young star
cluster models can be used, free from the contamination of any underlying
background stellar population. In most cases, however, the ionising population
is usually embedded in a large and complex system, and the observed photometric
properties result from the combination of a young star-forming burst and the
underlying older population of the host. Therefore, the second objective of
this paper is to provide a grid of models useful in the interpretation of mixed
regions where the separation of young and old populations is not sufficiently
reliable. We describe the set of PopStar Spectral Energy Distributions, the
derived colours and other photometric parameters, for mixed populations where
an underlying host population is combined in different mass ratios with a
recent ionising burst.
