  The properties of inhomogeneities on the surface of active stars (i.e. dark
spots and bright faculae) significantly influence the determination of the
parameters of an exoplanet. The chromatic effect they have on transmission
spectroscopy could affect the analysis of data from future space missions such
as JWST and Ariel.
  To quantify and mitigate the effects of those surface phenomena, we developed
a modelling approach to derive the surface distribution and properties of
active regions by modelling simultaneous multi-wavelength time-series
observables. By using the StarSim code, now featuring the capability to solve
the inverse problem, we analysed $\sim$ 600 days of BVRI multiband photometry
from TJO and STELLA telescopes exoplanet host star WASP-52. From the results,
we simulated the chromatic contribution of surface phenomena on the observables
of its transits.
  We are able to determine the relevant activity parameters of WASP-52 and
reconstruct the time-evolving longitudinal map of active regions. The star
shows a heterogeneous surface composed of dark spots with a mean temperature
contrast of $575\pm150$ K with filling factors ranging from 3 to 14 %. We
studied the chromatic effects on the depths of transits obtained at different
epochs with different stellar spot distributions. For WASP-52, with
peak-to-peak photometric variations of $\sim$7 % in the visible, the residual
effects of dark spots on the measured transit depth, after applying the
calculated corrections, are about $10^{-4}$ at 550 nm and $3\times10^{-5}$ at
6$\mu$m.
  We demonstrate that by using contemporaneous ground-based multiband
photometry of an active star, it is possible to reconstruct the parameters and
distribution of active regions over time, and thus, quantify the chromatic
effects on the planetary radii measured with transit spectroscopy and mitigate
them by about an order of magnitude.
