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 , H _ { \alpha } and H _ { \beta } 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 calculate the evolution of the cluster and the region geometry in a consistent manner .
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 ( SEDs ) , and the derived colours for mixed populations where an underlying host population is combined in different mass ratios with a recent ionising burst .
These colours , together with other common photometric parameters , such as the H _ { \alpha } radius of the ionised region , and Balmer line equivalent widths and luminosities , allow one to infer the physical properties of star-forming regions even in the absence of spectroscopic information .