The divas divide into two groups. The first group strove not to vary tone color for dramatic expression but to maintain consistency of tone color for the sake of musical line. Half the divas in the filmBarbieri, Cerquetti, Cigna, Pobbe and Simionatobelong to this group (as do virtually all singers today). From their point of view, a change in tone color compromised musical line as much as a break in legato. That they didn’t vary tone color didn’t prevent them from being emotionally intense. They relied on good diction and musicianship to serve librettists and composers.
For the second group, varying tone color for dramatic expression was paramount. Adami Corradetti (as a performer but not as a teacher), Frazzoni, Gavazzi, Gencer and Olivero are in this group. To my ears, these performers succeeded in changing tone color without damaging the musical line and thereby heightened emotional impact. The singers in the first group acted with their faces and bodies. The singers in the second group also acted with their voices.
One can find counterexamples. Frazzoni and Gencer didn’t always come alive interpretively. Cerquetti sometimes inflected her tone, most notably on a live recording of Ballo. Cigna on some occasions colored hers as well.