[en] The extent to which the use of different photometers can affect results on stellar color indices was investigated by simultaneously observing a sample of widely different stars with several uvby photometric systems available at the ESO, La Silla, Chile. In one instance, the ESO 50-cm telescope and the four-channel photometer at the Danish 50-cm telescope were used simultaneously. The reductions were performed separately on each data set, using a linear color transformation procedure. The results agree with theoretical investigations which showed that substantial errors can arise from the nonconformity of passbands. It is emphasized that it is necessary to use separate color transformations for various stellar types and classes and for different interstellar reddenings.