The scintillations of stars observed through the Earth's atmosphere are generated by random irregularities of air density. We propose the qualitative theory for description of coherency and correlations of optical scintillations measured at two wavelengths. It is based on a two-component model of air density irregularities: One of the components corresponds to anisotropic irregularities, while the second one is generated by locally isotropic turbulence. The main conclusion of the developed theory is that chromatic aberration results in low coherency of isotropic scintillations. The scintillations measured by GOMOS fast photometers (FP) on board the Envisat satellite have confirmed the theoretical conclusions. The coherency of scintillation measurements at wavelength 672 and 499 nm visualize the regions of high coherency where the anisotropic irregularities dominate. Observations have allowed also the detection of layers with low coherence. They are located generally between altitudes of 30 and 40 km. The thickness of the layers and their altitude distribution depend on observation location. It is expected that the locally isotropic turbulence is strongly developed within these layers. We show that the low values of the cross-correlation coefficient of two-wavelength scintillations can be used as a qualitative indicator for the presence of layers with prevailing isotropic turbulence. The obtained results showed that the analysis of two-wavelength coherency and cross-correlation functions is a sensitive approach which will allow visualizing IGW breakdown in the stratosphere.