  The Bayesian isochrone fitting using the Markov chain Monte Carlo algorithm
is applied, to derive the probability distribution of the parameters age,
metallicity, reddening, and absolute distance modulus. We introduce the
\texttt{SIRIUS} code by means of simulated color-magnitude diagrams, including
the analysis of multiple stellar populations. The population tagging is applied
from the red giant branch to the bottom of the main sequence. Through sanity
checks using synthetic {\it HST} color-magnitude diagrams of globular clusters
we verify the code reliability in the context of simple and multiple stellar
populations. In such tests, the formal uncertainties in age or age difference,
metallicity, reddening, and absolute distance modulus can reach $400$ Myr,
$0.03$ dex, $0.01$ mag, and $0.03$ mag, respectively. We apply the method to
analyse NGC 6752, using Dartmouth stellar evolutionary models. Assuming a
single stellar population, we derive an age of $13.7\pm0.5$ Gyr and a distance
of $d_{\odot}=4.11\pm 0.08$ kpc, with the latter in agreement within $~3\sigma$
with the inverse Gaia parallax. In the analysis of the multiple stellar
populations, three {populations} are clearly identified. From the Chromosome
Map and UV/Optical two-color diagrams inspection, we found a fraction of stars
of $25\pm5$, $46\pm7$, and $29\pm5$ per cent, for the first, second, and third
generations, respectively. These fractions are in good agreement with the
literature. An age difference of $500\pm410$ Myr between the first and the
third generation is found, with the uncertainty decreasing to $400$ Myr when
the helium enhancement is taken into account.
