On this question of colour in the voice, the mastery writer and critic Legrouvé says: "Certain particular gifts are necessary if the speech is to possess colour. The first of these is the Metal of the Voice. He who has it not will never shine as a colourist. The metal may be gold, silver or brass; each has its individual characteristic. A golden voice is the most brilliant; a silvery voice has the most charm; a brassy voice the most power. But one of the three characteristics is essential. A voice without a metallic ring is like teeth without enamel; they may be sound and healthy, but they are not brilliant. . . In speech there are several colours—a bright, ringing quality; one soft and veiled. The bright, strident hues of purple and gold in a picture may produce a masterpiece of gorgeous colouring; so, in a different manner, may the harmonious juxtaposition of greys, lilacs, and browns on a canvas of Veronese, Rubens, or Delacroix.
"Last of all the velvety voice. This is worthless if not allied with one of the three others. In order that a velvety voice may possess value it must be reinforced (doubleé) with "metal." A velvety voice is merely one of cotton.
"It may be of interest to notice that the quality which in France is designated "timbre," is called by the Italians "metallo di voce," or "metal of the voice." Those who heard Madame Sarah Bernhardt fifteen or twenty years ago will readily understand why her countless friends and admirers always spoke of her matchless organ as "la voix d'or."
Some singers control but two colours or timbres—the very clear (open) and very sombre (closed), which they exaggerate. In reality, however, the gradations between them can be made infinite by the artist who is in possession of the secret—especially if has the ability to combine Colour with Intensity.
Hallam, W. E. Style in Singing (1911): 15-17.