How achromatic is the stellar scintillation on large telescopes?

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.