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The properties of inhomogeneities on the surface of active stars (i.e. darkspots and bright faculae) significantly influence the determination of theparameters of an exoplanet. The chromatic effect they have on transmissionspectroscopy could affect the analysis of data from future space missions suchas JWST and Ariel.  To quantify and mitigate the effects of those surface phenomena, we developeda modelling approach to derive the surface distribution and properties ofactive regions by modelling simultaneous multi-wavelength time-seriesobservables. By using the StarSim code, now featuring the capability to solvethe inverse problem, we analysed $\sim$ 600 days of BVRI multiband photometryfrom TJO and STELLA telescopes exoplanet host star WASP-52. From the results,we simulated the chromatic contribution of surface phenomena on the observablesof its transits.  We are able to determine the relevant activity parameters of WASP-52 andreconstruct the time-evolving longitudinal map of active regions. The starshows a heterogeneous surface composed of dark spots with a mean temperaturecontrast of $575\pm150$ K with filling factors ranging from 3 to 14 %. Westudied the chromatic effects on the depths of transits obtained at differentepochs with different stellar spot distributions. For WASP-52, withpeak-to-peak photometric variations of $\sim$7 % in the visible, the residualeffects of dark spots on the measured transit depth, after applying thecalculated corrections, are about $10^{-4}$ at 550 nm and $3\times10^{-5}$ at6$\mu$m.  We demonstrate that by using contemporaneous ground-based multibandphotometry of an active star, it is possible to reconstruct the parameters anddistribution of active regions over time, and thus, quantify the chromaticeffects on the planetary radii measured with transit spectroscopy and mitigatethem by about an order of magnitude.

Correcting for chromatic stellar activity effects in transits with multiband photometric monitoring: Application to WASP-52