In this paper, the encoding of spectra is studied considering a variety of different observers on the one hand and a large set of standardized and non-standardized illuminants on the other. A number of 24 different observers has been defined including the CIE 1931 standard observer and the CIE 1964 supplementary standard observer as well as standard deviators. Others are selected with respect to largest differences from measurements published by Stiles and Burch. In addition, different illuminants are applied. Altogether, 52 illuminants are considered for reproducing color stimuli including such as D50, D55, D65, D75, A, B, C, E, and F1 to F12 as well as measured ones. So, this set contains illuminants with uniform spectral radiating power on the one hand and spiky power distribution on the other. A number of different encoding methods has been tested by calculating color encoding errors for all combinations of observers, illuminants and a representative set of test spectra assembled from data of Vrhel and Pointer. The encoding methods considered are based on expansions into basis functions derived from original or pre-distorted test spectra, Fourier and sine series as well as expansions into modifications of the latter ones. Maximum as well as average errors are presented.