  Context. The TESS and PLATO missions are expected to find vast numbers of new
transiting planet candidates. However, only a fraction of these candidates will
be legitimate planets, and the candidate validation will require a significant
amount of follow-up resources. Radial velocity follow-up can be carried out
only for the most promising candidates around bright, slowly rotating, stars.
Thus, before devoting RV resources to candidates, they need to be vetted using
cheaper methods, and, in the cases for which an RV confirmation is not
feasible, the candidate's true nature needs to be determined based on these
alternative methods alone.
  Aims. We study the applicability of multicolour transit photometry in the
validation of transiting planet candidates when the candidate signal arises
from a real astrophysical source. We seek to answer how securely can we
estimate the true uncontaminated star-planet radius ratio when the light curve
may contain contamination from unresolved light sources inside the photometry
aperture when combining multicolour transit observations with a physics-based
contamination model.
  Methods. The study is based on simulations and ground-based transit
observations. The analyses are carried out with a contamination model
integrated into the PyTransit v2 transit modelling package, and the
observations are carried out with the MuSCAT2 multicolour imager installed in
the 1.5 m TCS in the Teide Observatory.
  Results. We show that multicolour transit photometry can be used to estimate
the amount of flux contamination and the true radius ratio. Combining the true
radius ratio with an estimate for the stellar radius yields the true absolute
radius of the transiting object, which is a valuable quantity in statistical
candidate validation, and enough in itself to validate a candidate whose radius
falls below the theoretical lower limit for a brown dwarf.
