  The atmospheric scintillation of stars is the main limitation of the accuracy
of ground-based photometry of astronomical objects. This becomes particularly
noticeable for a study of a variability with amplitudes on the order to
thousandths of stellar magnitude or less. We examined the problem of the colour
scintillation --- fluctuations of difference between light intensities measured
simultaneously in two different photometric bands. Relations between the colour
scintillation power (index) and the atmospheric turbulence, telescope diameter
and the characteristics of the photometric channels are derived. Asymptotic
dependencies for large telescopes (1--10 m) are obtained that allow to predict
the value of the colour scintillation for a particular telescope and detector.
It is shown that the colour scintillation index is ~D^{-3} for measurements
with both short (milliseconds) and long (seconds) exposures. The impact of the
atmospheric dispersion which amplifies colour scintillation away from the
zenith is estimated. We show that colour scintillation in the long-exposure
regime depends strongly on the wind direction in the upper atmosphere.
