When sighted from the ground in clear weather, stratospheric clouds make large changes in the sky color during twilight. Spectrometric measurements performed from the ground during CHEOPS show that the color changes can be either large reddenings or blueings. A radiative transfer model demonstrates that the first are caused by thin hazes above 22 km while the second are related to thick clouds below, that the color change during twilight is little sensitive to tropospheric clouds. Statistics of presence of PSC's above Kiruna during CHEOPS which shows that reddenings are correlated with temperature below NAT condensation at high altitude (30 hPa, 21.5 km), support this interpretation, as also the consistency with PSC measurements with in-situ aerosol counters, balloon radiometer and from satellite observations. Spectrometric observations of the zenith sky at twilight is therefore thought to be a powerful method for identifying the presence and the altitude of PSCs above a station in all tropospheric weather conditions. ¿American Geophysical Union 1991