Changing tone colors is one of the basic skills a singer should acquire. It is particularly important in singing medleys, where a singer must switch tone colors almost instantly, during a few bars between songs, according to what the content of each song requires. Singing with clear changes of tone colors, however, is not an easy task. In seeking to attain it, I tried application of the oreille electronique used in the Tomatis Method, by matching impression of each song with the pass-band of a certain language. This has proven to be extremely helpful in producing the intended changes. I suggest that such results can also be explained in terms of the Bel-Canto technique, a term widely recognized by vocal experts as a synonym of "the ideal vocalization." One of the qualifications of the Bel-Canto is "consistency of voice texture." Once different types of texture are established, they are aggregated into various tone colors. Thus, the Bel-Canto pieces require the skills to keep and change tone colors freely. My assumption is that these skills coincide with what the Tomatis Method, with its purpose being improvement of vocalization by way of refining auditory senses, offers to accomplish. My discussion starts with reconstructing the concept of the Bel-Canto, comparing it with the Tomatis Method and illustrating their similarities. It then moves on to showing a model of tone assignment based on the Tomatis Method. A solo version of the chorus medley "Four Seasons of Japan" is used as an example. The new way of assigning tones demonstrated here, I believe, offers a promising alternative to singers striving for better command of their voices.