The "silent substitution" method, which has become well-known mostly through the work of Rushton and his collaborators, can be traced back to experiments performed by M. Ishihara under Exner's supervision at the beginning of the century. Rushton provided a theoretical framework for the method with the enunciation of his "principle of univariance". In this paper we show how the "silent substitution" concept can be further generalized to any arbitrary number of photoreceptor classes by making use of well-established concepts of colorimetry. With this approach, which we have called "spectral compensation", one also gains a better insight into the possibilities and shortcomings of the technique. To illustrate this, we apply our approach to examine a number of published studies where use has been made of "silent substitution", with particular emphasis on the work of W. A. H. Rushton.