  The Sun and stars with low magnetic activity levels, become photometrically
brighter when their activity increases. Magnetically more active stars display
the opposite behaviour and get fainter when their activity increases.
  We reproduce the observed photometric trends in stellar variations with a
model that treats stars as hypothetical Suns with coverage by magnetic features
different from that of the Sun.
  The presented model attributes the variability of stellar spectra to the
imbalance between the contributions from different components of the solar
atmosphere, such as dark starspots and bright faculae. A stellar spectrum is
calculated from spectra of the individual components, by weighting them with
corresponding disc area coverages. The latter are obtained by extrapolating the
solar dependences of spot and facular disc area coverages on chromospheric
activity to stars with different levels of mean chromospheric activity.
  We have found that the contribution by starspots to the variability increases
faster with chromospheric activity than the facular contribution. This causes
the transition from faculae-dominated variability and direct
activity--brightness correlation to spot-dominated variability and inverse
activity--brightness correlation with increasing chromospheric activity level.
We have shown that the regime of the variability also depends on the angle
between the stellar rotation axis and the line-of-sight and on the latitudinal
distribution of active regions on the stellar surface. Our model can be used as
a tool to extrapolate the observed photometric variability of the Sun to
Sun-like stars at different activity levels, which makes possible the direct
comparison between solar and stellar irradiance data.
